Originally Posted by
atalose 
Trying to figure out solutions for her problems is not on your shoulders. Her moving in with her son is not a healthy solution but that is between them and has nothing to do with you. It’s not even your place to suggest it.
I might suggest instead of focusing on her drinking or not drinking issues that you focus on your codependency and inability to stand up for your own needs and wants.
I totally, 100% agree with this advice. At the same time, I know it can be tough advice to follow.
Last year, I made the mistake of letting my A sister-in-law move into our house "temporarily" while she was going through a tough separation from my ABIL (spouse's B). Had I known the extent of her drinking, I would not have allowed her to stay with us for even a night. Our eyes were opened in the weeks after she moved in (around Halloween) and it became clear that we could not let her stay indefinitely. We made the conscious choice to let her stay through the holidays but began trying to talk to her about leaving in January. When the "nice" approach did not work (i.e., she started trying various manipulative tactics to stay), I had to be more direct.
I knew that I was doing the right thing by asking/requiring her to leave, and I knew that if I could manage to be crystal clear with her that she would go. That didn't dispel the guilt that I felt. So, in order to feel "right" with myself, I went online (roommates dot com and some other sites I don't recall anymore) and printed out a handful of rooms for rent that were cheap, local, and potentially reasonable options for her. My point was not to "find" her a place to go, but to show her that she had options and to point her in some direction.
Frankly, there were some good options out there - I had no idea that such cheap and nice-sounding accommodations even existed.
I gave her the printouts along with a note that said (in part), it's time for you to move along, you have X time to find another place to stay, here are some ideas to get you started.
She chose not to pursue those options or anything similar, instead crashing at friends' houses and eventually turning up back at ABIL's house causing great chaos to the entire family (including young children). She blamed me for kicking her out "without notice" and with "nowhere to go" - neither of which was true - but I felt zero guilt over the situation at that point. I felt that I had done something reasonable to try and help her, and her failure / refusal to step up or help herself was not on me at all.
I guess my point is - great if you can let your sister's problem be hers to solve without feeling guilt - but if that's not working then consider what if anything you can do to "help" her that won't ask more of you than you can take on. Giving her some resources may work for you as it did for me (it also may help you in a more direct way, because if you know specifically that she has other viable options then there's no reason to feel such responsibility for her).
Sorry for the long post, but I had to tell the whole story as I'm still amazed how well this strategy worked for me - not that it helped her at all, but it was an "easy" way for me to resolve my own discomfort with a tough situation.