My Dexter sometimes picks on his sister too. I think its sometimes when he's bored rather than real agression. So yes like Gill says maybe a toy would distract him. But maybe as Bonnie is approaching adulthood, Bebe is making sure she knows who's boss?!
It hasn't been serious so far, but yes I alway pick one of them up & put them away from each other. I think thats just an instictive reaction!
Has she JUST been spayed? As sometimes after a stay in the vets cats can be quite off with each other as they smell funny
A lady I know had that problem quite seriously between her 2 cats after one of them had a lot of vet treatment (after about 15 yrs of living with each other as well). She now has a 'rota' system where one is confined to one room for half the day while the other is free to roam the rest of the house. Nightmare!
Hopefully it will settle, but cats do have spats, as long as there's no serious damage done.