I Never Knew

My younger sister was in a lesbian relationship with a really attractive woman. I am a straight male and blink and I would have sex with you. Well her partner and I got tipsy and she said she would like to try anal sex. Nothing happened but I did sober the next day look into Google and found what to do. I am not some anal expert on the subject.
It's not that we party all the time but I did end up drinking with my sister and her partner and when my sister went to bed the subject came up again. I probably was the one to introduce the subject but I don't remember.
I had anal sex with her and she loved it and wants more.
I did not like it and I did not cum and I love my sister.
I can't get over doing this to my sister.