What to do about an immature little sister?

I'm the mother of two adorable girls, Allegra 6 and Adria, newborn. I love them dearly and would do anything to protect them. What if it means from family though? My little sister (shes 25) and has had a hard life. She was ually abused by her gym teacher in middle school, has been arrested for possession (she did drugs to cope with the shame ), and she hang out with men who take advantage of her, emotionally, ually, and financially (they even hit her). I have tried to help her repeatedly, by letting her live with us, getting her into rehab, and bailing her out of jail on that one occasion. However, she is on a serious rode of self-distruction. I don't want her or her boyfriends around my daughters. The only problem is that they love their Auntie Sofia. I don't want to be the bad guy and I love my little sister a ton. She's been through a lot. My husband wants her out of Allegra and Adria's life though. What to do?