INA Section 245(I) states in pertinent part: After three years, the U alien, and even a spouse, child or parent who did not receive U status initially, may adjust to permanent residence." Further, aliens granted T and U petitions by USCIS and their immediate famiy members may obtain visas abroad, even in the first instance, and may adjust status to permanent residence in the U.S. 9 FAM 41.84 NN 1 and 4. The form is I-929.
Your mother, brother and you are of course U visa holders. Now your stepfather is not your biological father. He is not your "immediat relative" so perhaps he would have to wait and get his green card later from your mom, once she get's it, and than file for him, but its not very clear also from the language of the statute above. Perhaps other colleagues on this site can chime in or know more. Good luck.
Now that your mother has had U-1 status for three years, she and the rest of you are eligible fto apply for permanent resident status (I-485). If you do not do that, you could be deported when the 4 year U status ends.
Your mother is eligible to file a petition (Form I-929) for your step-father. If she demonstrates that either she or your stepfather would suffer extreme hardship if your stepfather is denied a green card, USCIS can allow him to apply for a green card at the same time as the rest of you even without being granted a U visa. He will not qualify for U status because the marriage must have existed at the time of filing of the original application.
Probably your stepfather would have his petition approved. I think USCIS is sensitive to the situation where someone is a DV survivor and finally finds someone who treats her with respect.
This is a particular focus of my office's practice.
I agree with the counsel above
My name is Stephen R. Cohen and I have practiced over 38 years and can be reached at 213-819-1171. I practiced mainly in Los Angeles and Orange County, California. I am not seeking clients from existing relationships with other attorneys, and give only limited advise over the phone (the phone is primarily used to set appointments), these services do not create an attorney client relationship. I apologize for mispelling< as I am a lousy typist, My answers may offend as I do not believe in pulling punches or sugar coating the truth. Further regarding courts in other states my opinions are largely based on logic and what I think is the modern trend which is to consider the needs of the child.