You should consult with a lawyer regarding the best legal way to resolve this issue. Immigration Laws are enforced at the borders, not the place one should wait to find out about if there are any "...additional serious risks ..." resulting from US PR abandonment.
I represent parents in DCPP a.k.a DYFS cases. Each defendant is appointed an attorney by the State of New Jersey if you can't afford one. Consult with an attorney. This is not the forum to address allegations and other issues related to your DCPP/DYFS case.
It depends on what you mean by "raised." What kind of relationship the children have with their biological father? Your soon to be husband ahould consult an attorney that specializes in child custody issues. He has to address his legal issues himself. You don't know all the facts so you can't seek legal advise for him.
Need more facts. Based on your research and the information you have given, I am assuming that your brother is over 21 years old. You know that your mother's petition, if approved, will be years before your brother if filed at the same time. What exactly is your question? Is your brother disabled and need your mother to be there to care for him or does your mother suffer from a medical condition that only your brother can offer her the care she needs? Otherwise, it is just a personal...