My girlfriend and I have two three-year-old cats: Napoleon (male) and Josefine (female). They are siblings from the same litter. I have been studying abroad for the last 9 months, so currently my girlfriend's the one taking care of them.
The two cats get along just fine for the vast majority of the day. They eat together, the female frequently bathes the male (though baths usually become a small fight for some reason, and she really doesn't like it when he tries to bathe her), and my girlfriend sends me pictures (and videos) of the cats sleeping cuddling together in the most adorable ways at least once a week.
However, whenever the two play together, Napoleon seems to have a hard time understanding the boundaries of acceptable behavior and takes things a bit too far.
It doesn't help that Napoleon is significantly larger (1 kg = 2 lb heavier even though they're the same age) than Josefine, and that she sometimes honestly seems to ask for it.
As a representative example, please watch this 45-second video. Josefine waits to ambush Napoleon as he leaves the bathroom (where we keep their sandbox) and gives him a light tap on the head (I interpret this as a play-ambush). He gets seemingly startled and then the situation momentarily escalates (but not severely), followed by Josefine becoming slightly submissive (hunching down) and letting out long, trembling meows as both seemingly vie for an opportunity to strike. Napoleon then lets out some meows of his own and Josefine becomes very submissive, lying down.
The video ends there, and my girlfriend says that's when she interrupted the fight by calling their attention. In similar situations when she hasn't stopped them (she usually lets them solve things out for themselves), Napoleon will go for a bite or something of the sort, Josefine will bat him away and run away, at which point Napoleon will sometimes give chase. If he does, the fight continues with more jabs and some biting. Nails always remain sheathed and the bites don't seem to be meant to injure (no one's ever come out bleeding, shall we say).
Sometimes we just see the chase and don't see the instigating incident (or whether there was one), but Josefine seems to be the usual initial instigator. Other times the cats play "tag", running around the house with no issue.
Once the fights are over, the cats don't seem to avoid each other or anything indicating lasting animosity. That being said, one time (and only one time that we're aware of) Napoleon did end up with a tuft of Josefine's hair in his mouth.
Since neither cat seems very affected by the fights, my girlfriend usually doesn't interrupt them. That being said, we'd rather stop this behavior or at least understand it better.
Is this actually just normal play-fighting (and the time Napoleon got a mouthful of hair was then just a one-off accident/escalation) to be shrugged off? Is our interpretation of Josefine's behavior incorrect, and she isn't play-fighting at all? Or, if we are correct in our diagnosis that Napoleon simply doesn't know how to play-fight, is there some way to teach him to turn it down a notch?
For some further context into our cats' recent history and how their behaviors changed (quite significantly) after I left to study, please see this other question I asked 7 months ago. This type of fighting also only started after I left, so that context might be relevant. (Also note that the issue raised in that question has passed)