I would say yes it is perfectly fine to charge them rent. Everything cost money, not to count everything you are doing for her. Don’t feel guilty either to take the money, you deserve that and more with the responsibility you have on your hands. As well, may I add, if there are any other syblings, don’t let them tell you what to do or not to do. She is under your roof and care, not theirs. Lots of luck !!!