dying inside

Well i been dating this guy for a while and i took my sister out to eat and she told me that she has a crush on this guy named Trevor but the bad part is im dating her crush and i didnt know of i should have told her oder not i sagte nice his cute but now when im with Trevor i think of my sister and i just feel really bad because she never had a bf cuz all the guys she liked they end up liking me and she gets mad but i never like the guys she like but she likes my bf and she doesn't know he is