I realize this is much simpler said than done, but try to put your focus exclusively on the people you're fellowshipping with at church so that the friction with your sister will be less dominant in your mind. If you're perpetually worried about her, you're not going to be able to give of yourself to others you're engaging with, absorbing everything they're telling you, or derive the benefit of talking with them. If she's brought up in a conversation, say as few words as possible about her, and salt them with grace. Then change the subject. During the service itself, devote your attention to it.
I'd keep any interactions with her while at church very minimum, and very light. Be as kind, polite, and brief as possible.
It's unfortunate there's tension between you and your sister. I like to think of my mind sort of being like property I own; I get to decide how much space someone occupies inside of it. If someone is consuming your thoughts, it's sort of like letting a tenant who doesn't pay rent and trashes the space remain there. It takes a lot of mental discipline, but you can train yourself to pull your thoughts off of her and put them somewhere else. When you're at church the best place to put them is on God and on the people there to worship. Perhaps memorizing a few scriptures that are important to you, and repeating them to yourself in your mind several times when you find your thoughts diverted to your sister would be helpful.