Why is my cat messing with the other?

I have 3 cats. Frank, Knarf (Frank backwards) and Catsia (female). Frank and Knarf are brothers and Catsia is a stray. We’ve had the brothers for quite a few years now. We’ve had Catsia since the Winter. If it matters, Frank was here first by a couple of months. Knarf is obviously the alpha, Frank seems to be a... show more I have 3 cats. Frank, Knarf (Frank backwards) and Catsia (female). Frank and Knarf are brothers and Catsia is a stray. We’ve had the brothers for quite a few years now. We’ve had Catsia since the Winter. If it matters, Frank was here first by a couple of months. Knarf is obviously the alpha, Frank seems to be a little bit on the feminine side, he keeps to himself, and Catsia seems to still be a bit curious it seems like. Catsia also keeps to herself. The brothers used to fight a lot it seemed like at one point, but they’ve mellowed out only fighting once in a blue moon. All three of them are fixed and still have front claws. When we first brought in Catsia, we had her in the garage and over a couple weeks gradually introduced them before letting Catsia in for good. Frank and Catsia don’t really get into it. About once a week, maybe twice every now and then, Knarf will attack Catsia. I’m worried about this because when Frank and Knarf get into it, neither of them whelp, or make a sound of distress. On the other hand, I could be playing on the PS4 and in the kitchen behind me, I’ll hear a sound of distress from Catsia. Next thing I know, Knarf is bolting it from the kitchen, across the living room, into one of my sisters rooms.

What’s going on, how could this be fixed, what’s the reasoning?
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