I almost got caught looking at my girlfriend's sister but that doesn't stop me.
Why do you keep doing it?
The sensation of getting caught makes it more arousing
That's unfortunate that you have those feelings.  Not only should you not be ogling other women when you're already in a relationship, it's doubly bad that it's your SO's sibling.  You need to keep your eyes to yourself.
Fantasies are not a crime. It's not a big deal and I don't believe that love is about ownership.
That's not where the problem lies, though. Looking at your girlfriend's sister that way is hurtful to someone you should love and protect. When you're noticed, and it's likely you will be, it would be damaging to your relationship, not only with your girlfriend, but with her entire family.