Cat with attitude

My cat has a real attitude problem. She used to be the sweetest kitty, I don't understand why she turned this way. I have two other cat's and if they come near her she has a fit. They are her brother and sister to her. Now she is pregnant and I'm wondering how nice she will be to her own kittens. She will walk throught the house and growl the whole time. Can anyone help me? She let's me hold her but she still growls. Once in a blue moon I can get her to purr.. If someone would give me some suggestions I would greatly appreciate it...

It is a very difficult question to answer without any background. Are they pure breeds part of a breeding program? How old are the cats? How long have they lived together? How many times have they been allowed to breed?

Just wondering if its possible she may be in pain? Now that she is pregnant, she may be going through some different mood swings. Or she could be anticipating having kittens & not liking the fact that the other two will be around bothering her, & possibly her kittens? Does she appear to be in discomfort?