I openly say I dislike one of my sisters and still hold a grudge towards her
Why do you still hold a grudge towards her?
I'm sure she likes to steal my things because she is always acting fishy.
It's really bad for you health to hold a grudge. It's also very rude to say stuff that can hurt your sister's feelings.
I don't care. I do what I want.
It's not right to say that your sister steals your things when you don't have any proof, and holding a grudge against her for it doesn't make any sense. I think you should stop treating her that way unless you're sure she's stolen your stuff.