I think it is perfectly fine to help family, friends, or strangers financially if you WANT to. If you want to help her, that is absolutely your choice, OP. In my family, we do help our relatives at times and no, we don't have to, but we want to do it, and I don't think that makes us pushovers or someone people take advantage of.
If you want to give her money for select things that you choose, just be aware that it will make you a target for requests on other areas of "help. "When those requests come to you, I would stop engaging her on that topic after her first request and your refusal. If she brings it up a second time, "SIL, we've already discussed this and my answer has not changed." If she brings it up again, then warn that you will end the conversation if she has nothing else to speak about - then do it.