Help!!!

Hello everyone! I haven't posted here before but I have been lurking for a little while!

So to get to the point, we have 2 cats, Petunia our 1 yr old spayed female, and Pepper, our 9 wk old little boy. Tonight when I got home, Petunia was attacking Pepper! She has never done this before and I was wondering if she could just be hot and cranky. She is long haired and it has been 90-100 degrees here lately. She still acts like she wants to play with him, but then she starts hissing and slapping him. I was going to call the vet and take her in tomorrow-- I just didn't know if that was possible. I know I have been cranky lately with the heat and I don't have long hair all over my body!
Thanks in advance,
pipersjo

You know I am sure there are other people around these board that can give golden nuggets of advice. But just from my experience when a new cat comes into the household, it takes time for the other cat to accept the new kitty. It could take days or months. I know when I got the new kitten I thought Tigger was going to hurt the poor new baby, but I guess in the cat world nature knows what she is doing. I fed them seperately for a while,and seperated them when I wasn't home and Tigger would slap the baby and of course the baby (Eddie) just took and came back for more. As long as there isn't blood then it's just natural kitty getting to know each other stuff. I know the Feliway plugon has helped our cats but please be patient. I am sure she will be a great big sister but right now she doesn't want to share her stuff (typical big Sis)

I will also say the feliway is great... It does take time to adjust....

Thank you everyone! I think what actually was going on is, my older cat doesn't like my boyfriend and so, every time he was near her, she would get mad-- and take it out on her little brother. My bf is not home now and the 2 cats are running around upstairs playing like normal (they sound like a herd of horses!).
Thank you so much,
pipersjo