Girlfriend’s sister got naked in front of me and I don’t know what to do

I don't know how to frame this. My gf and I have been together for almost 3 years now and she is a twin and her sister has been single since I met her. I constantly found myself going out to bars with them and my gf's sister can't handle her liquor. She gets sloppy drunk almost everytime.(I've always been super respectful and careful because I know as a man things can go south quick) Basically have had to carry her and my gf when they get too drunk. One day during a parade they both had too much to drink and I took my gf and her sister to the bathroom. My gf was throwing up and her sister was making advances towards me and calling me "handsome" and she wishes "I was hers". After my gf threw up I helped her up and was helping her brush her teeth, her sister gets naked and rubs her tits on me and grabs my hand and quickly puts it on her butt. I didnt notice initially, I turn my head slightly and my heart dropped. Id be lying if I said she didn't have an amazing body but I never saw her that way. I jumped in the shower and hid behind the curtains. I told her to put her clothes on after she did i got out quick. They both don't remember this and I dont know how to tell my gf about it and if I even should