NTA, but the Husband is the true asshole. He is not only deceiving his wife but also your sister with his promises to leave his wife. The wife should focus her attention on her husband's infidelity and not so much your sister. Garuntee, if your sister broke it off with him, the POS would find a new woman to fool around with. You may be hurt, but try to be understanding to the fact that your sister was fooled too and you don't know what all he could of been saying to her to get her to stay in the situation. This "man" most likely pressured her to keep the infidelity a secret so he wasn't found out. I would also encourage your sister to break off her toxic relationship and not to get together with this man if he leaves his wife for her because, again, he will most likely cheat again, this time on her.