At my oldest brother's wedding, my mother spent the entire service sniping (and not so quietly) to my grandfather about how the bride wasn't "good enough" to marry her son - because of who her father is!
I was 18 at the time and quietly horrified, could see my brother getting more and more flustered and angry as he stood in front of us waiting for his - actually very lovely - bride to make an appearance, and could not convince my mother to shut the fuck up! My father couldn't shut her up, either, and my grandfather was just sat there; not encouraging her in any way, but not doing anything to stop her, either.
Turns out, when they were growing up, my mother and the bride's father were involved together for a while when they were mid-teens. Until he threw my mother over for the girl he eventually married - the mother of the bride. And my mother never forgave him for it. So when my oldest brother announced he was getting married to "THAT girl" (said with disparaging expression and sneer of contempt every time), my mother was distinctly unhappy about it. She spent the morning of the wedding trailing my brother and his best man around the small town he grew up in - from barbers to cafe for one last fry up as a single man - haranguing him into not turning up at the altar, jilting the woman he loved, doing as Mummy told him to... (he was 32 and his bride was 23 at the time, mind). She and my father had travelled up the day before the wedding, and I didn't arrive until mid-morning, the day of, because I had an 'A'-level exam the day before - I remember walking into their home, and seeing my father and grandfather polishing shoes and asking where my mother was. "Oh, she's with [brother] in town," says my grandfather. "We told her not to go, but... you know what she's like!"
Well, if I didn't beforehand, I certainly did that day!
Luckily, she didn't leap to her feet when the priest asked if anyone had any objections... but it was a close-cut thing. My grandfather and I both turned and glared at her, at that point, and I think she thought better of any dramatic actions (thank God, because I suspect my brother might actually have done her harm, if she'd objected!). However, at the reception, she refused to sit at the top table, forbade my father from doing so (my grandparents - who actually raised my brothers - sat there, instead), dominated all of our elderly great aunts and uncles so that my brother and sister-in-law barely got to see them (and it was the last time all 10 siblings of that generation were together in the same place 
), interrogated my then-boyfriend about our sex life as we were trying to eat the wedding breakfast (the youngest of my great-uncles - who was around 75 at this point - was sat next to me and he just leaned over and said "everyone does it, it's not like she invented it and doesn't like to share now, is it, lovely?"), flirted non-stop with my brother's best man (whom she's known since they were toddlers), and just generally riled my brother up to the point where every photo we have of him and his bride (first dance, etc), he looks absolutely furious.
He and my sister-in-law went NC with my mother after that.
No one blamed them one little bit.