My 3 are 12, 10 and 6.
The older two are very close and always have been. They really enjoy spending time together and play together a lot. Youngest wants to play too, but she's too young to fully understand the game and is quite headstrong and demands they play to her rules and basically makes the game no fun at all for the older two, so they push off without her, or refuse to play her games. She is regularly devastated when this happens.
Sometimes I think they are a bit harsh with her, but I try not to always leap in with pleas to let her play. I don't want to forever be defending her, because although she's my baby and I wish they'd be a bit more lenient, I can also see that she's their annoying little sister and as much as they love her, she isn't always much fun to spend time with. I bit my tongue a lot, but perhaps not enough as the older two roll their eyes when I do try to stick up for youngest.
No one is actually being victimised or unkind to anyone. It's just a mismatch of age and personality.
Youngest and middle child play nicely when eldest isn't around. Youngest and eldest play together occasionally, but not often.
Any experience, thoughts or advice?