If you have no idea how to fix and limited resources, you may find a couple of books handy. Do you have a library available to you? You'll need a plumbing and carpentry book. You absolutely have to open the kitchen ceiling and find the leak source! It will also allow you to assess all of the wood structure including the bathroom subfloor and joists. I would hope the joists that hold up the entire floor are not rotten. If they are, I hope your mother has some equity in the home because you will need a professional to fix that level of damage. But if the joists are solid you could pull this off with some books to instruct you and perhaps some friends to help. Pictures would help considerably. Well lit, focused pictures.