My girlfriend is on vacation as part of her college course. Her sister sister came by to our apartment to hang out i have a great relationship with her family. Anyway she told me she could drink me under the table and i couldn't refuse her offer. That's all i remember because i blacked out shortly afterward. I woke up the next morning with a terrible hangover and rolled over in horror to see her sister naked next to me. She informed me we had had sex last night and better yet it was unprotected sex. I started freaking out and asked her if she was joking. She told me she was dead serious. I'm so upset with myself i don't know what to do. My girlfriend and i have been together for 4 years and i recently bought her a ring to propose to her. I have always valued the honesty in our relationship. But i don't know if she can handle the news i slept with her sister. In my defense i was drunk and don't even remember having sex with her. Should i tell my GF?