DP and I are getting married next year.
My parents are currently getting divorced, they separated 12 years ago. My dad had several affairs and generally treated my mum badly. I have a strained relationship with him but I know he loves me in his own way.
My dad has had the same girlfriend for 14 years (I'll call her Susan). She was the catalyst for my parents splitting up, although if it wasn't her it probably would have been someone else (my dad is a seriel cheater and I wouldn't be surprised if he cheats on his Susan)
I really don't like my Susan. She has tried hard to be friendly towards us and always encouraged my dad to maintain a relationship of sorts with us. But she bad mouths my mum at every opportunity, including to me and also to my sister.
My sister had her wedding a few years ago when things weren't so tense between us and Susan. Susan got drunk and made a dick of herself and did a few things to make my mum feel like second best (if I say what I will probably out myself). Since then Susan has done a lot of bad mouthing and shit stirring and I find it so hard to tolerate her.
I genuinely don't know what to do. For all his faults, my dad is still my dad and I wouldn't want to upset him. My sister refuses any contact with Susan and only sees dad on his own but I know she would understand if I invite her. My mum has always behaved with dignity in spite of Susan's horrible behaviour to her and again, I know she will understand if I invite her.
This is a geniune AIBU. The invites haven't gone out yet so I have some time to consider what to do.