JD2019

Proof that you were both under 18 years old when your parents got married = parents' marriage certificate and your and your step's birth certificates to show you both were under 18 when they got married.
 
Proof that they are still married; i.e. are not divorced.  Statement from them if possible that they are still married.  

For each sibling you need I-130 the form itself Proof of US Citizen: copy of passport or naturalization  Proof of sibling relationship: copy of birth certificates to show the common parents (plus translation) Some countries have hold-house record/book system: get a copy (plus translation) Some other secondary supports: pictures, phone calls, mail/email, etc. Check/Money Order/Credit Card Authorization Form: $535 No I-485 cannot be filed until the priority date becomes current: unless you filed I-130 like more than decade + ago, you cannot file it now. Income requirement and financial support documents are not needed now but after 12+ years after petition is submitted.

Hi
 
neither. the i485 is for adjustment of status, which isn't the case, they don't qualify for adjustment of status in the country
 
the document you need is a copy of each other's birth certificate, with translations into English to prove you are siblings
 
the wait can be 14 years or more, up to more than 20 depending on their country of origin
 
no need for tax returns for now. file the i130 for each sibling, with payment and documents. and then forget about the petition. it will take many years for the i130 to be approved.