How to stop a Bully Boy

My boy Finn is bullying his sister Darci(both 2 years old same litter). It is driving me crazy. Half the time they are friends - they come downstairs when I come home together they sleep on the ottoman together but in the two months or so he is chasing her around in the evening.

I heard I am supposed to let her protect herself but I can't stand to see him bullying her so I have her come sit with me - doesn't always stop him but I chase him away. I actually think he may be being playful. She just growls and hisses so she is not happy. Often I try to distract him with toys or petting etc... sometimes it works sometimes it doesn't. I hate to see her sleeping pacefully only to have him come up and hit her. Oh and she is not completely innocent either sometimes she reaches out and swats him but it is 80/20. They are about the same size too. Should I keep them apart? Use Feliway? Any advice? Help - please

what does she do when he comes and hits or or "attacks" her? Does she cower down and take it, does she fight back, run off?? I would try Feliway. I assume they are both neutered?

she fights back a little and gives up then she runs off and he chases her and both were fixed when they were 5 months or so.

that sounds more like play and an occasional hiss or growl in the mix somewhere is normal too...