I don't really have any good advice. I too have had problems with 2 females fighting. Luckily one was my mom's cat Tribble, a female and my cat Deirdre another female. I say luckily because we were able to keep them seperated and that kept them from hurting one another. We had this while system of sharing the main part of the house between the two cats and whos cats got to be out in the main part when. The other set spent that time in the bedroom of the owner, either mom's or my own. But then we still had problems because neither wanted to back down in territory. So they were peeing on places in the main part of the house marking. So we ended up building a wall across the main portion and split the house in half basically so that they don't ever have to cross one another's territory. That pretty much works. But none of that will really help your cousin. Perhaps someone who has integrated more cats together will have some advice.

Denyce