I haven't read all the replies, but I've never heard of someone not adopting out sibs before.

I have Pooky & Bear (brothers I got at 4 weeks old) and they sometimes get on each other's nerves but I wouldn't say they fight. Bear is a cuddler and will snuggle up with Pooky and nap.

I have the 3 sibs (Sassy, Sabrina, Josh) 2 females, 1 male. They do sometimes actually fight. Mostly when someone goes to the vet and smells different. The fighting is usually instigated by Sabrina (named at 2 weeks old after a witch mind you since she's a torti) and Josh (who is just a bully when he's upset). None of the 3 of them ever truly cuddle, but will sometimes wash each other, or lay next to each other without incident. They do play together but sometimes it gets too rough and I have to break it up.

I think it's more of a personality thing myself.