Don't know what to do(please answer)

So my fiance has a younger sister (10 years old) she's been thrown around from home to home her whole life (in between the family though) because their mom is a druggy (no contact with her father) their mom has been clean for about a year but relapsed recently so my fiance's sister is now with an aunt again, anyways my fiance and I are considering getting full custody of his sister but we don't know if it's a good idea since we are both very young and starting college etc. I really want to take her it's not good for her to just go from home to home, she's such a sweet girl I'd hate to that go away 😢