Why do they act this way?

This is the third time I have caught my newer cat, Autumn washing my other cat Lily. As soon as they see me watching them, Lily acts all ticked off at Autumn and starts to bite and slap at Autumn until Autumn gets annoyed and starts to fight back.

Before I enter the room the two apparently are lovey-dovey. Anyone else have cats that fake you out?

hee hee my boys do that...LOL
I think it is a sibling rivalry thing
Lorien

Oh, yes! Well, the two in question are RB kitties, but yes.

Shasta was an only kitty for fourteen years, and then Gryphon and Nibs came to our house. Shasta wouldn't ever admit to us that she was friendly with either of the guys, but it was not at all uncommon to happen upon her and Nibs grooming and generally being friendly. And, like your kitty, she instantly became very gruff with him.

Appearances, I guess -- mustn't let the humans know that you approve of this interloper