I'm not going to get my sister an engagement present
Why won't you get her a present?
I don't like her that much, I get on her nerves a lot of the times
While I agree that you should not expect gifts for you wedding, I still think that you should reconsider. It is almost a custom to get a couple a gift when they are getting married.
Fuck them. I don't like being around them for more than one hour.
I'm sorry that you don't like them, but I still think you should at least give them a small gift. It's an important event in your sister's life.