How do I tell my girlfriend that her sister is hitting on me?

However you do it, do it with care. First of all, be 100% sure that her sister is hitting on you. Some people are just naturally flirty and you wouldn't want to make a false claim. If though, you are sure that she is laying it on thick, you need to speak with your girlfriend directly. Chances are she's going to be upset about it so make sure you remain on her side and give her lots of reassurance. Your sister may or may not address the subject with her sister so you need to be prepared for it to happen again. Decide how you're going to react before it actually happens, and make yourself perfectly clear to her that her advances are completely inappropriate.