I would say much the same as Frankies Girl, except that I would start with consulting the lawyer, to find out just what your recourse is if he doesn't quickly and immediately agree to either start paying rent or buy you out or put the house up for sale. marble_faun's question about his financial situation is a good one - does your brother have the financial means to buy you out, or would he be able to get a mortgage? I'm not sure that I'd want to get into a "rent to own" situation with family, but if you do, make sure it's with legally drawn up paperwork and the whole 9 yards!
Also, you haven't said what your relationship otherwise is like, and you certainly don't have to share that much about your family relationships on an internet forum. But I journeyed very closely alongside someone in a similar (though not identical) situation and the outcome in terms of relationships was quite devastating. Not just for the people immediately involved, but in the wider family circles. I do hope you're prepared for the possible backlash, if you decide to push ahead to have the situation change/resolve. Not saying you shouldn't do it, just something you should think about. And be prepared for how it might also stir up feelings about your mother's death, and your childhood relationships, etc. Sometimes these situations have deeper "roots" into our souls than we realize...