Cat beating up on his sister

Hello, I have 2 cats, Nigel (8 month old male) and Lloyd (3 year old female). They really love each other for the most part, always sleeping together and cuddling, cleaning eachother. But Nigel is way too rough with Lloyd. He will start chasing her around the house, and then dig his claws into her to the point where she is hissing and screaming at him. I cant tell if he is just playing and doesnt realize that he is being too rough, or if he is doing it on purpose. Lloyd doesnt stand up for herself either. Please help! Im sick of him beating up on her like this.

Has this just started?  Or has this been their normal since they have been living together?  Also, how long have they lived together?

I'm wondering--just off the bat--if something is triggering it, such as him seeing another cat outside, smelling another cat, something causing misdirected aggression.

Thank you for the reply! They have been living together since Nigel was about 5 weeks old, so about 7 months now. Now that he is almost her size, I would say this has been going on for the last few months. They are both indoor cats, but there is a few cats in the neighborhood that come into our yard that I know Nigel and Lloyd both watch through the window. I'm watching them right now, and Lloyd just attacked him first (in a playful way), and then hissed. So maybe I am just over thinking it all. I just want them to be bffs!

Is Nigel neutered?  If not, you may have a festering problem on your hands!  It is very similar to the way an unaltered male reacts to intruders on his territory...although I have a 12 year old that has been neutered since he was 1 year old, and he is still EXTREMELY territorial, however I suppose age has brought wisdom, and he doesn't take it out on his roommates if he sees another cat now out the window, or smells one through the door crack.

There are a ton of posts on here about misdirected aggression....put that into the search bar and read through a few, see if it sounds like familiar behavior to you.

If he isn't neutered, I would highly recommend doing so ASAP so your girl doesn't become his whipping post!

My 3 do play pretty rough and tumble, and I have a 19 pounder, a 13 pounder, and a 4 pound girl, 13 years, 3 years, and 2 years respectively...my old guy gets too rough, mainly because he doesn't see it as play--he's all business and doesn't want to be bothered, but even my young two when they play-fight don't hiss.  The hissing is a "back off and QUIT" warning.  But, that being said, some cats hiss over everything.

They are both fixed. I honestly think it is nothing more than hI'm just getting a little too carries away. After all, he is still a kitten. And Lloyd is very calm and independent, while Nigel is a lover, always cuddling on my lap and sleeping with my husband and I every night. Maybe their two personalities are just very different. I'm hoping he will grow out of this the older he gets.

Wow. I really butchered that! It's supposed to say "him" and "carried" haha

It is VERY possible.  All 3 of mine have very different personalities.  The old guy is a pure grouch and doesn't want any part of play, unless, of course, he starts it.  Then he gets too rough when the others join in the chase, and somebody usually squeals because he has gotten too rough, and the party stops!

So it's probably something just like that. Thank you for the help! I need to let him just enjoy his kittenhood Have a great day.

She may decide to teach him some manners haha!  Good luck, and enjoy the kittenhood while it lasts!