Excuse the long post but it's hard to explain and I'm not the best with grammar either!
I recently got engaged to my partner and we've been discussing our wedding with family members, I was having a discussion with my mum that I wished for my dad to be at my wedding, for context they've been divorced for 10 years after she found out he'd been having an affair with one of her friends. Me and my dad haven't always had the best relationship due to me being a spiteful teenager and taking my anger out on him and his partner about the divorce. We hardly saw each other when I was teenager due to me refusing to see him. I know he was wrong for having the affair but he's still my dad and I've chose to finally have a relationship with him, over the last 2 years we've become close and I see him regularly as well as his partner- the same women who was once my mums friend. I mentioned I would be inviting him and his partner to the wedding as I'll be having a small wedding and will not be inviting anyone else from my fathers side and can't stand the thought of my dad sitting alone at my wedding as I know none of my family will speak to him! She's thrown a massive hissy fit and had told me she won't come to my wedding if dads partner is invited and that I shouldn't bother to send her an invite.
I'm now stuck in a dilemma as me and my mum have always been close, I see her 3-4 times a week and she has my children to sleep over regularly. What would you do? Invite mum and have dad sitting alone which would break my heart or not invite mum which would also break my heart not having her at my wedding. 
Sorry for the long rambling post but I can't sleep as it keeps going round in my head of what I should do!