Why is my cat acting out towards new people?

Try this and I hope it helps:

http://www.absolutelycats.com/22Kitty.html

Good Luck!

Is your cat spayed? She's acting very territorially - are you spending less time with her? Did the sister move in to the cat's room?

Does the sister spend time where there are other animals? The scent would be very threatening to the cat.

You need to reassure the cat. Hold her, pet her, speak soothingly to her, while YOU sit with the sister and talk or watch TV. Hold her tight, securely, so she knows she's safe. If she struggles TOO much, don't get hurt, let her go. But you want her to associate you and happiness with the new resident.

Also, try rubbing some catnip on your hands, see if that calms her down. If it does, have the sister do the same - and on her jeans too. The cat will see her in a new light.

If the sister is spending time with other animals, she needs to change clothes when she comes in, and keep the clothes away from the cat.

Imagine being in a house with your boyfriend, and having him come home every day with different expensive perfumes wafting around him. You can talk, the cat can't, you'd definitely have things to say, right?

Good luck. This will take time, repetition, and therefore patience and cooperation from you and your roommates.

It could be just a matter of feeling like its an invasion when others are around. Some cats are like that, with a few breeds having a high propencity for being territorial and possessive of just a select few people.