Yuki should be spayed, the sooner the better. This could be a result of hormone fluctuation, and she could be coming into heat? It is not healthy for cats 6 yrs. old to be continually having heat cycles and is detrimental to their health and may result in a lot more expense.
It could also be behavioural. It's possible something happened between them to upset her. Perhaps she is jealous of Tigger now for who knows what?...maybe your catsitter paid more attention to Tigger than Yuki. And she doesn't like you intervening & saving Tigger from her attacks. You might try giving Yuki extra attention when she is behaving well, feeding her first before the others, grooming her if she enjoys it, playing with her. It you can catch Yuki in a preparatory mode before she attacks Tigger, try to redirect Yuki's attention into an activity she likes instead (throw a toy, have a treat bag handy to shake). Keep us posted, but do get Yuki & Molly spayed (no longer a kitten) above all.
Last edited by catlover2; August 21st, 2009 at 11:29 PM.
Reason: Correction: Yuki is unspayed; Tigger is spayed.