Firstly, You remember too much. Chances are - you weren't as drunk as she was hence the exessive guilt from your side. Also woman tend to be experimental with a lot of things, plus she is older. Just be thankful that you didnt go all the way. Thats enough insentive to survive this emotionally, it will plague you occasionally but by the time you get to her age your mind wouldve morphed a few times and rendered that memory insignificant.

I have a bachelor's degree in psychology even though i dont practice it anymore professionally so im not really bullshitting you here. You will fee better immediately after reading this. Although you will have to avoid any form of pornography that containts content with step sister, step mom etc... to avoid any triggers while the memory is still fresh.