I would have to agree with your theory. Have you been gone since your mother got home? Just curious because you say it was mostly when you were gone that Pepper would sit on your mother's knee. Tubby is like this also. If I'm around, Tubby will always come to me and, for the most part, ignore Terry. But if I'm not around, Terry tells me that Tubby is all over him, bumping on him, climbing on him and wanting pets from him. Tubby is not jealous of Terry, but he is extremely jealous of Peanut, so it is possible for a cat to be jealous. I'm sure Pepper will get back to normal, it might just take a little time.