Both my parents and my fiance's parents are divorced, all of them are remarried (twice, in my fathers case), and none of them get on.
My parents had a really messy divorce when I was 6 that didn't really end until i was 8. My dad remarried when I was 11, and had another three kids, but they divorced five years ago, and my dad's now been seeing his new girlfriend for a year now. I'm not close to any of them (my dad, former-step-mum or my dad's girlfriend) but I do love my dad and my former step-mum, so I'd never think of not inviting them to the wedding (along with my dad's girlfriend, of course). My mum also remarried a lovely man five years ago, and I'm actually pretty close to him. My mums parents also helped raised me too. However, my dad really struggles to be civil to both my mum and former step-mum, he can't be civil at all to my grandpa and new step-dad, and his girlfriend avoids my mum and former step-mum at all costs.
My fiance's parents divorced when he was 9. His dad remarried a woman with couple of kids of her own when FI was 13, and she was really vile to my fiance and his brother when they younger, and his dad never really did anything to stop that, so they don't really get on, but FI hopes this will be a chance to reconnect with his dad. His mum (who is amazing) remarried at around the same time, but FI is really close to his mum and step-dad. FFIL and FMIL have only seen each other twice since they got divorced, and last time they were in the same room together, FFIL and his wife got into a fight with FMIL and her husband, and ended up leaving early.
So my question is, how do we make this work? We are paying for the wedding pretty much by ourselves (his mum & step-dad, and my mum's parents are giving us some money towards the wedding), and I've known since I was a teenager that if I got married, I wanted my mum to walk me down the aisle.