Do you think it is okay that I find my girlfriend's sister to be hot?
Do you find your girlfriend's sister hot because of her appearance?
Her glasses make her quite sexier
You should not be unfaithful to your current girlfriend. You need to focus your interests on the person you are with or you will find yourself alone because your girlfriend won't like this.
You are no fun and I don't care.
While sometimes it can make sense to find other women hot even though you're in a relationship, you shouldn't look at your girlfriend's sister in that way. Trust me, you could lose your girlfriend and her sister could end up hating you if you keep playing such a dangerous game.