I have 3 dds aged 9, 7.5 and 3.5. The older two are quite close and have been good playmates for a long time. They can argue quite a bit too but love playing together just the 2 of them.
The youngest is now old enough to play but she's often left out as the older two see her as a bit of a pain, she plays with stuff then they can't find it etc. When the older two aren't playing together they'll often read for ages (both keen readers) which is obviously great but quite a solitary activity.
I feel I spend the afternoons after school fielding dd3 away from her sisters so they can play/read in peace. But from dd3's point of view, she's missed them while they've been at school and wants to be involved. BTW she is at nursery and is starting to have friends of her own.
AIBU to expect them to play with the little one sometimes? Sometimes I ask them specifically to be met with complaints that they 'always' have to play with/look after her.
I just shouted at the older two today as they both accused me of having a favourite (dd3) which of course is rubbish and really hurt me as I do try very hard to be fair and spend time with each of them individually too.