My birthfather is in Nigeria and gave up rights to me and my sisters when he and my mom divorced. My mom subsequently married my step-dad when we were 12, 14 & 16 years old. My stepdad raised us in his house both before they were legally amrried and of course afterwards. Our mom died a few years later when we were 14, 16 & 18. Our stepfather continued to raise us, but never legally adopted us. He took care of applying for social security benefits from our mom for us and basically was the only parent we had. All 3 of us eventually moved out on our own after becoming adults. I moved back in with my stepdad about a year ago as he is 98. He now passed away and did not leave a will. He left minimal money but does have a house on a decent piece of property. He has no known living relatives (parents, siblings, first wife and only bio-child have all passed away before him). Do I have any rights to his estate? And what happens to his estate now?

That is a tough one. Since you weren't adopted nor part of the bloodline then depending on your states laws it's a coin toss. The argument can be made that he held you as his own. And clearly he was a decent man to keep you after your mom passed. So it may be there is a chance for inhieritence. Again its going to depend heavily on the state you live in.