If she works a job, I would say to tell her if she is going to stay with you she has to put every extra penny into building a fund to be able to move out on her own, even if it is just renting a room somewhere. Make it clear the ultimate goal is for her to move out. You cannot carry on this way.
This is a wonderful idea, it also gives you the power, makes you less of a "bad guy" as you don't want to look (that will change with counsel you) I think when you do this you will feel relief and empowered. You just might wonder what took yourself so long. It is your home, it is your rules, and it is your peace.
Bless you, for taking care of your grandchild! Wanted to throw that in.
Most importantly, what I want to say: I don't know if you feel guilty about your brother, if you don't good, If you do it is not your fault! Not!! so.... if part of your sister living with you is guilt or fear it might be again.... it's up to her, not you. The longer she stays with you in this condition, the harder it will be on you and your family, and the worse she will get.
You can do this. Keep coming back here, I say this to everyone when I post; these people here are amazing.