Timona

Yes

Sounds like they want a signed letter of intent to marry within 90 days of US entry from the beneficiary.  That's all you should send. 

Mom will have to show proof of name change with marriage certificate from the third marriage. Did she use her second husband’s name? I believe when you complete the DS260 you have to show all names used AND proof of name change.
Mom might receive an RFE for proof of termination of all marriages at the NVC stage. I don’t think your mother will be able to move to the US until she sorts out the second and third marriage. 

When your Priority Date (date you applied) is before the date on Table B you can start the second part of the process (pay fees, submit docs to NVC, complete the DS-260). But there's still no visa available to you at the point. Table A is when a visa becomes available to you, so when your Priority Date is before that date you'll then be scheduled for interview and actually able to get a visa. As you can see from the Visa Bulletin, that date is currently June 2020, so people who applied before that date are able to now get visas - so they've waited just under 4 years. 
 
The wait may speed up or slow down, no way of knowing unfortunately as it will depend on how many people are in the queue ahead of you. All you can do is watch the VB (it's released monthly) and see how the date is progressing. But yes, you may find that your husband can become a USC before your PD becomes current. If that is the case, then you will be processed straight away, as spouses of USC's are classed as immediate relatives, so don't have to wait.
 
Good luck. 

Where are you right now? In the US or overseas?
Did you submit the I130 and I130a online and only those documents or did you submit additional forms? 
ETA: where did you say you were going to interview in the US or in another country? 

So she was provided a reason.
 
Is this the same extensive immediate family that you have, yet you are here.

How did you and your sister get to the US? 
Are either of you US citizens yet? 

Normal interview.  All individuals applying to visit the US are already assumed to have immigrant intent.  The responsibility for overcoming that intent lies within the information on the DS-160.  It is especially hard for parents of US citizens.  She will have to satisfy the IO's assumptions that she won't return to PI.
She was, almost certainly,  denied due to lack of strong ties to her home country.  That is the #1 reason for B2 denials.   It is at their total discretion.

She would have been told why she was denied they would have given her a piece of paper citing the relevant clause of the INA.

Sounds a normal interview. You mean your mother applied and you helped I assume.
 
What is a mother-in-title?
 
And what would you be calling about?

Don't withdraw anything. I pointed out that you need to do. Re-read my post.
 
I don't think you'll be able to get a final divorce decree in time to reply to USCIS. Hence why I edited and explained that respond with the papers from the court that says you filed divorce.
 
As soon as you get that final divorce certificate, send it with a copy of your I-751 receipt notice to the address at the bottom of the receipt notice.
 
I see for VA, you're eligible to file divorce after 6 - 12 months of separation (depending on circumstances). Well, you've exceeded that. So, file that divorce today and get it rolling. 
 
In the meantime, if you have 60 days or so to respond to RFE, then I believe you can just wait to get final divorce. However, if you don't have much time/ anticipating a fight with ex-hubby, then resort to paragraph 2 above.

There's nothing like "we love each other/ communicate daily/ every second" but live separately. Nothing of such fairy tales exists in USCIS dictionary. 
 
1. File for divorce or
2. he moves back in/ you move to him.
 
Those are the only options you have. Otherwise, your case is going to be denied. How does your husband move to SC and not back to you in VA? Sounds like this marriage is already dead but you're holding on for GC purposes. 
 
File divorce. You have no option.
Submit:
1. Divorce filing (the one that says you filed divorce. It'll buy you time).
2. RFE letter 
3. Amended I-751, with divorce option checked. 
 
You'll still have the GC, albeit correctly and wih less stress. 
 
USCIS already knows you guys live separate and no letter nor explanation can overcome this, more so with him preferring to move to SC.
 
@Family is one of the more tender/ optimistic Vjers. So, if she says your goose is cooked, it surely is. 

There is no sponsorship for a B2/tourist visa.  You can help your sister with money for the trip (transferring funds to her bank account), and to fill out the application (DS-160), but beyond that, it is her application and will rely on her ties to her home country. 
 
Good Luck!

There's nothing like "we love each other/ communicate daily/ every second" but live separately. Nothing of such fairy tales exists in USCIS dictionary. 
 
1. File for divorce or
2. he moves back in/ you move to him.
 
Those are the only options you have. Otherwise, your case is going to be denied. How does your husband move to SC and not back to you in VA? Sounds like this marriage is already dead but you're holding on for GC purposes. 
 
File divorce. You have no option.
Submit:
1. Divorce filing (the one that says you filed divorce. It'll buy you time).
2. RFE letter 
3. Amended I-751, with divorce option checked. 
 
You'll still have the GC, albeit correctly and wih less stress. 
 
USCIS already knows you guys live separate and no letter nor explanation can overcome this, more so with him preferring to move to SC.
 
@Family is one of the more tender/ optimistic Vjers. So, if she says your goose is cooked, it surely is. 

What are you referring to? Affidavits from friends or I-751?
 
If I-751, it's your case. Your signature will be the only one needed if you choose the divorce waiver option. 
 
If affidavits from friends, they're weak evidences. Resort only to them if you lack strong evidences. 
 
Buena suerte ☘️
I'm out. 

Don't withdraw anything. I pointed out that you need to do. Re-read my post.
 
I don't think you'll be able to get a final divorce decree in time to reply to USCIS. Hence why I edited and explained that respond with the papers from the court that says you filed divorce.
 
As soon as you get that final divorce certificate, send it with a copy of your I-751 receipt notice to the address at the bottom of the receipt notice.
 
I see for VA, you're eligible to file divorce after 6 - 12 months of separation (depending on circumstances). Well, you've exceeded that. So, file that divorce today and get it rolling. 
 
In the meantime, if you have 60 days or so to respond to RFE, then I believe you can just wait to get final divorce. However, if you don't have much time/ anticipating a fight with ex-hubby, then resort to paragraph 2 above.

There's nothing like "we love each other/ communicate daily/ every second" but live separately. Nothing of such fairy tales exists in USCIS dictionary. 
 
1. File for divorce or
2. he moves back in/ you move to him.
 
Those are the only options you have. Otherwise, your case is going to be denied. How does your husband move to SC and not back to you in VA? Sounds like this marriage is already dead but you're holding on for GC purposes. 
 
File divorce. You have no option.
Submit:
1. Divorce filing (the one that says you filed divorce. It'll buy you time).
2. RFE letter 
3. Amended I-751, with divorce option checked. 
 
You'll still have the GC, albeit correctly and wih less stress. 
 
USCIS already knows you guys live separate and no letter nor explanation can overcome this, more so with him preferring to move to SC.
 
@Family is one of the more tender/ optimistic Vjers. So, if she says your goose is cooked, it surely is. 

Really? And they told you this? You must be new to immigration. 
 
Your question could have been asked within the first 2 lines. 

Even hiring a lawyer will require you to do the legwork with respect to gathering the documents needed to file.  You will need all the divorce decrees to file for new spouse.  As to your first husband if he knows his A Number that should be fine, you shouldn't need all the documents you used to file for him all those years ago.
 
Good Luck!

Give them all evidence of you working on your marriage.
They are thinking you and your husband only stayed married so you could obtain an immigration benefit (10 year GC and eventually citizenship).
If health insurance and life insurance were in your name- and your husband was not working at the time- it could be viewed as part of the agreement- you pay for his health insurance in exchange for the 10 year GC. 
Do you have additional proof from the previous 921 days of your continuing relationship?
e.g scheduled visits, vacations together, therapy sessions, communications about where you will live together once he figured out his mental health, communication about long term plans to reunite?
 
I would include all communication and proof of your marriage- right now you write about your husband like he is just a friend- someone you married to give health insurance while they gave you a GC. You need to show proof the marriage was real and not that you married him just for a ticket to the US. 
 
ETA: Affidavits from friends are not the strongest evidence. You need solid documentation and third party letters from people who are not friends and family- as @Timona mentioned before- affidavits are weak evidence. 

You'll be filing / amending I-751 to get conditions removed on your own. Your spouse won't be signing it anymore.
 
However, if you're on good terms, you can get a statement from your spouse describing how your relationship started, progressed and ended. In detail. He'd also say he doesn't object you getting conditions removed. You'll write a similar statement.
 
I'd also explain what took you so long to divorce and efforts in the past to save relationship. If you have proof of any marriage counselling sessions etc - that would help.
 
Good luck!
 
 

I think you are making a big mistake of you try and justify the last 4 ish years.
 
Be grateful they did not approve your case without an interview.
 

What are you referring to? Affidavits from friends or I-751?
 
If I-751, it's your case. Your signature will be the only one needed if you choose the divorce waiver option. 
 
If affidavits from friends, they're weak evidences. Resort only to them if you lack strong evidences. 
 
Buena suerte ☘️
I'm out. 

What are you referring to? Affidavits from friends or I-751?
 
If I-751, it's your case. Your signature will be the only one needed if you choose the divorce waiver option. 
 
If affidavits from friends, they're weak evidences. Resort only to them if you lack strong evidences. 
 
Buena suerte ☘️
I'm out. 

There's nothing like "we love each other/ communicate daily/ every second" but live separately. Nothing of such fairy tales exists in USCIS dictionary. 
 
1. File for divorce or
2. he moves back in/ you move to him.
 
Those are the only options you have. Otherwise, your case is going to be denied. How does your husband move to SC and not back to you in VA? Sounds like this marriage is already dead but you're holding on for GC purposes. 
 
File divorce. You have no option.
Submit:
1. Divorce filing (the one that says you filed divorce. It'll buy you time).
2. RFE letter 
3. Amended I-751, with divorce option checked. 
 
You'll still have the GC, albeit correctly and wih less stress. 
 
USCIS already knows you guys live separate and no letter nor explanation can overcome this, more so with him preferring to move to SC.
 
@Family is one of the more tender/ optimistic Vjers. So, if she says your goose is cooked, it surely is.