From the adults perspective - my mum and dad divorced over 20 years ago. It was a nasty, spiteful, horrendous split on both sides which resulted in my siblings and I going into care due to cruel false accusations that left both sides looking unfit to parent. Both my parents families said all kinds about each other and when my mum remarried a couple of years later my step family got dragged into the mess as well. Everyone pretends to have moved on now but there are still little comments that give away people's true feelings - there's still bitterness and resentment festering away, particularly between my aunties on my dads side and my mum.
My mums family unfortunately do not get along particularly well with my step dads family (because they are a bit snooty which I admit is irritating but isn't a reason to leave them out).
Both my mums family and my dads family is huge (several generations of lots and lots of kids) so I have 3 Large groups of relations that I dread attempting to put in the same room together.
Then there is DP's family. His mums side is absolutely massive, his brother had over 200 guests at his wedding just on his side, not including the brides family. They have a lot of family politics going on and the bride and groom were tearing their hair out over the seating plan beforehand. DPs dad died when he was quite young and although his mum and dad were engaged at the time of his dads death, his dads side cut him and his brothers off after their dad died so while they are sort of in contact now I'm sure you can imagine the kinds of things that have been said over the years and his mums family are still aggrieved that the boys and their mum were treated this way. So 2 more family groups to dread putting in the same room together.
None of his family have met any of my family because they live 300 miles away, so there won't be any ice breaking beforehand (not particularly relevant but I'll include it anyway, we also have the challenge of finding a venue that everyone can afford and get to).
DP and I are seriously considering taking DSD on holiday to Vegas and just getting married while we're there!