Brother and sister cat keep fighting

[googoodolly]

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.

Any advice or similar stories??

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.

yes, I can see that ours are still playing too. they still curl up and sleep together so it's not hateful fighting.

we can't let them out unfortunately.

Our two cats do the same brother and sister. Have hissed at each other as well. One top cat for a bit then change s whose top cat. Next minute they licking each other and snuggling up.