Nowhere near as bad as some of these, but I went to my DCousin's wedding a number of years ago - my aunt and uncle (DCousin's parents) had split up when he was 18 months old due to his DM's extra-marital affair. My Aunt then moved on to the Other Man as soon as they'd split up and my cousin ended up being mostly raised by his dad and aunt (his mum's sister) as his Mum was too busy shagging her new man to look after her own child.
That's a bit of background to explain why we were so confused on the wedding day in question.
Other than the Aunt that helped raise my Cousin, the entirety of the rest of the family on the DM's side completely blanked my uncle and us. It was like the breakdown of their relationship was somehow his fault, even though she was the one that cheated and effectively abandoned her own child.
In fairness to my cousin's mum, she was actually pretty pleasant on the day - it was the rest of the family.
We spoke briefly to my cousin's maternal grandmother, who in those 5 minutes managed to throw in numerous derogatory comments about my uncle (that I quickly and politely shot down before my own DM wasn't as polite!).
I felt so sorry for my Uncle - it was his child's wedding but he was left on the sidelines and had to put up with people giving him a death stare throughout his speech (yes - father of the groom speech!). His new wife had decided not to attend, which I'd thought was odd when they told us, but given the "welcome" that my uncle (and we!) had received, it was totally the right decision.
Thankfully for my cousin, he didn't seem to pay any attention to it, and clearly adores his dad and made plenty of time for us too. He just either didn't notice, or didn't want to rock the boat with the other side of the family.