Older cat bullying younger cat

[Chrsprs156]

I have 2 cats who I adopted from the rspca in February aged 6 (milly) and 11 (Katie).They weren’t badly treated their previous owner unfortunately died. They were both got as kittens by their previous owner.
The older cat is bullying the younger cat, katie hisses and pounces on milly and swipes are her just for milly walking into the room that Katie is in. When milly is eating Katie stares at her until she walks away. Milly doesn't show any aggression to Katie. They don’t have to compete for affection as there are 2 of us in the household and we love them them both very much.
Does anyone have any advice for what I can do.