3 year old cat, behaviour

Question
I have 2 cats, one 13 year old male (Sharife), and a 3 year old female(Sushi), they are both fixed. They have always gotten along very well, yesterday night I heard unbelievably loud growling and hissing, so I went over to go see what was up, and Sushi had her ears pinned and was growling hissing and striking at Sharife, I tried to get her to stop but she wouldn't so I picked up Sharife to get him away from her, and she followed me continuing her behaviour, after about 5 minutes I brought them both back downstairs and sat with them and they were fine so we thought maybe they were just having a fight, but this morning around 6:30 the same thing re-occured, and now shes acting this way with my brother as well, though she's fine with everyone else,I'm starting to get really worried, do you have any ideas on what could be going on?

Answer
Mariah,

It seems to me that Sushi may be seeing a strange cat new to the neighborhood.  This may bring out a fight/flight/fright reaction.  If this happens, the cat often loses control and does not recognize her usual buddies.

When this kind of thing happens, the best thing is to take Sushi and put her into an interior room where there are no windows until she calms down.

Please let me know if this behavior continues or gets woprse.

Best regards... Norm.