For background: attended Friend A's event with mutual Friend B, event was held at Friend A's parent's house, lovely big place, lots of family and friends, huge buffet.
Friend A has a nine year old DD. A's brother is quite a bit younger than her, mid twenties. He's very energetic. I don't know how else to put it?! He wouldn't leave the DD alone, teasing her verbally, this then escalated into putting his foot out to trip her, chasing her etc. At one point he actually had her on the floor, "just messing about" but essentially wrestling. She was on her back, he was sitting on her, arms pinned, and was going "try to get up, go on, try to get up". His parents and older relatives were smiling indulgently, and Friend A looked a bit awkward but said nothing. The little girl was saying 'ow, you're hurting my wrists" which made him lean down harder and tickle her with the other hand. She started to do that laughing crying thing, he finally let her up, and you could see she was upset and holding back tears. He started mocking her as being a cry baby, her grandmother then said "oh, she's just so oversensitive" and comments were made on how she was an only child, unused to ordinary rough and tumble. The little girl had a book with her, she kept trying to retreat into a corner to read, but was repeatedly hauled out by her uncle, and then was scolded by her mother and grandmother for being rude, and not engaging with people.
At one point I went to the bathroom, and the poor little thing was hiding in a sort of alcove off the hall, with her book. I told her I wouldn't tell 
she came back down eventually, and there was more goading and teasing, reducing her to tears, whereupon she was told again that she was spoilt and touchy.
There were two other children at the event, one older and one younger, belonging to Friend A's other older brother, and they didn't get any of this.
This is absolutely nuts, isn't it? A grown man wrestling a young girl down to the floor and sitting on her while other adults look on, saying nothing? I was completely shocked and discussed it on the way home with Friend B, who thinks that it's a bit weird but normal in the context of some large rowdy families. I'm an only child and I have two children myself, and we are all quiet and introverted, so maybe she has a point, but AIBU to think this child is being bullied by her uncle while her family watch? If a grown man tripped my daughter up, wrestled her to the floor, then sat on her, he'd be exiting the building on stumps.