Niece and nephew

OK so my brother baby's mama always ask me for things for her kids. (My niece and nephew) I recently took in my niece paid for her driving school got her back in school and provided for her while she lived with me. It turned out to be a lot of drama so I had her go back with her mom. I never asked anything for my kids but than again my brother is good for nothing. But she has also been known to take advantage. I work part time and do what I can. It just makes me feel bad when I can't. Any advice on how to handle this?