nohappyending

Start clearing his $@@$ from the closet to make room for hers and he’ll catch on.

There's an old saying that is somewhat to the effect of "no house is built big enough for two women."
 
If things are tense with your MIL now, it's not bound to be much better when she's actually living in your home and probably "taking over" or commenting on everything from how your husband is fed, how the home is decorated, your parenting skills (if you have children), your cleaning skills, whether you're a good wife in her eyes, etc.  In other words, I can see the large potential for her to want to continue mothering him as though he were still a child.  And then, there's the complete lack of privacy---which, as a still relatively newly married couple, you're going to miss greatly.
 
My first question to him would be "why didn't you even discuss this with me first?"  If his response is something along the lines of "it's my decision---it's my mother", then I'd probably (personally) respond back with "it's our house---not just yours".  Maybe ask him to put himself in your shoes---if roles were reversed, how would he feel if you planned on immigrating a relative to live in the home and didn't even deem it a necessity to discuss it beforehand?
 
I'd also have a game plan before any process begins if you do decide to go ahead---is she workable?  Is she going to be self-supporting in any way or will you both be paying all of her expenses, and causing more financial strain, going forward (cellphone, clothing, entertainment, personal effects, transportation, etc.)?  Is her living with you going to be temporary---she'll have her own apartment once earning enough of an income---or she's in your home until her dying day?  All things that will need to be discussed.
 
I also wonder...considering he didn't even tell YOU, has he even broached the subject of immigrating his mother with HER, or is this just all a pipe-dream of his right now?

Why does he want to bring his mother so badly?  I wonder if figuring out what is behind his thought process will help you find some areas to negotiate.

Also, have you looked into how long this is likely to take him?  Do you have the necessary household income to sponsor her?  You might discover there's something that will make it difficult.

Sounds like you have some serious relationship problems to work on, if he does not respect your opinion on these important issues.  If he chooses his Mom over you it says a lot about his true intentions in marrying you.  By the way, your name isn't Chantel is it?

He can file for his mother.   When you are asked for a I-1864A, that will be another discussion just as to where she will live.  Hopefully she isn't too old to earn enough credits for medicare and social security.

The way you asked your question seems more like you are seeking family/relationship advice rather than immigration advice.
 
Immigration wise, you can not stop him from petitioning anyone that is eligible. If he decides to go through with it, he can do so without you. As a US citizen, he doesn't need you for anything immigration related.
 
If you two share a home, you DO have a say who is allowed to stay in your home. That is about the extent I can offer you advice on how to deal with you issue outside of immigration.
 

At least you dodged the bullet on getting sued base on I-864.
I-134 filed during K-1 visa is non legal binding.

There's marriage aspects, legal dissolution of marriage, that are STATE specific.
Then there's the immigration side of things, as well. Usually folk seek to withdraw the I-864 before the green card is issued, this puts full stop to the Adjustment of Status Process with USCIS.

Well I guess since you do want to wok things out and you feel he may just need a space in time it seems to let things just play out.
Let the immigration process continue let him get his green card. He wrks all that stuff. Hopefully you can sit and talk and work things out. Hopefully this is just a adjustment thing you know.