We have two 20 m/o kittens. They came from the same litter and were fine up until a couple of months ago. Both are neutered and are indoor cats.
Boycat has tons of energy and loves to play, but unfortunately this is aimed at his (timid and shy) sister. He pins her down and bites her neck and she has started fighting back. No major injuries yet but it's only a matter of time.
We have 2 sisters around the same age, their fights have got more vicious recently but I can still tell they are "just" playing. They play worse if they've been stuck in the house all day. They're predators and need to go outside and hunt.