My thoughts do go out to you Diana, especially as thread has turned into a bit of a spat between two. Not helpful.
I have an elderly friend who has for years dealt with an alcoholic grown-up daughter. Nothing they’ve done has helped. They’ve endured physically very abusive boyfriends, theft, prison, repeated paying for rehab. He is a broken man. His wife will still not give up on her, but it is their daughter.
Personally, I would evict your step daughter but get her in touch with any useful agencies and her brother before you do. Or your twilight years will end up as sad as my friend’s. He was such a lovely sociable person. Now he refuses to go out.
Understand you feel an obligation to your late husband, but ultimately she is not your daughter and I think it’s time to think about yourself and your future.