I'm the human of two female young cats, one is 7mo the other around 5mo, they're wonderful and behave very well, very affectionate, they like it around us and feel confident around the house. They also play with each other with no aggression or nasty behaviour.
But here's the thing, and I can't find anything about it online. If I play with both of them at the same time, the elder will absolutely refuse to play if the younger is playing. As soon as I pick the little one or remove her from the room, she will get into hunt mode immediately. If the younger is involved, she will remove herself from the situation and just look at how the other cat plays... No aggressiveness, no complains, just switches to passive mode.
It's not major but I'm a bit sad because whenever I play with them I need to separate them otherwise just one of them plays.