My sister met a man on a dating app and has been seeing him for nearly a year. He claimed to be separated and living in an annex of the family home for financial reasons, but just about to move out and file for divorce. Over time his story has changed - there's no annex, not even a spare bedroom for one thing (he and his wife have 2 teenage children who live at home).
It turns out he and his wife have been allegedly separated but living this way for the past 6 years after they both cheated on each other. He let it slip that his wife despises him. Yet he says he's going to live there for another 2 years to help his wife because then she'll be able to start claiming a pension (even though they're only in their 50s and she's never worked much this is his current story).
My sister is vulnerable she suffers from mental illness including depression. She is really into this guy and it's got to the point where she's afraid to ask him too many questions. Our parents are worried but they actually don't dislike the guy and I also must admit in some ways he's very good for her. He's encouraging her to take evening classes and to work on her art and music and to get therapy, for example.
He spends every weekend at my sister's flat and they also go on mini breaks together. He even mentioned they're thinking about getting a cat! They act like theyre in a normal relationship. She has of course never been to his house or met his wife and children.
I'm not sure how to respond when for example they ask if my husband and I want to have dinner with them, etc. I don't want to enable this relationship but at the same time I don't want to alienate my sister. Also how on earth is he doing this - what could he be telling his wife? The whole situation is bizarre, isn't it, or is this quite common?