I have a 23 year old Siamese who doesn't like anything or anybody except me. She hates my new cats who are brothers that I recently adopted from a shelter. She growlsand hisses when she sees them. Any chance she will ever just ignore them?
Your Siamese is the oldest cat I've known. Maybe she is an astidious lady and the new cats made her feel inconvenient. I think she is not young enough to accept the competition. I think you'd better try to console her more.