DS recently had a birthday, and my sister offered to host the party (we were getting work done on the house, and my sister has a big garden with enough space). Invited both sides of the family, so my parents and DHs parents and siblings. They've all met before, but don't know each other all that well. However, my sister does a similar job to 2 of DH'S sisters, so they have that in common.
Anyway, party rolls round, we all get to my sister's house. ILs barely spoke to my sister the whole time. They just stuck together in a little group. My parents and sister all tried to chat, but were pretty much shut down. Then SIL happened to go into the kitchen and did get talking to my sister. They chatted for 20 minutes or so. But then SIL went out, rejoined her siblings, and said "There, I've done my bit, I talked to one them. It's someone else's turn now.". She didn't realise I overheard.
I thought this was really rude, but when I mentioned it to DH just laughed and said "Yeah, we don't like talking to people". These siblings are in their 30's, so not teenagers who might be shy. I was quite annoyed, as it felt like she was sneering at my sister and almost making fun of her by pretending to have a conversation that actually she didn't care about.
Would you think this was rude?