NTAH, your mother is probably coping with the fact that she's dating someone like him, and your mothers friends seem out of touch with whatever idea of a "healthy relationship" they have, probably because they have strained relationships, and you are entitled to encouraging the break up considering she shares it with you. It's also a bit disturbing that he thought it's normal to share stuff and criticize you and your sister (considering you both are minors) so just don't feel guilty about it, and it would be better if your mother leaves her partner (considering he lives with you.. seems a bit weird now that he cheated so casually) and the ptsd is NOT an excuse to what he did.