Oh Shit!

Last night I came in from a three-month long trip to Europe, and during my welcome back party at my sister's apartment she and I got seriously hammered.  This morning I woke and, as you might have guessed, both my sister and I were buck naked in bed together. 

Neither of us have any recollection of the night before, nor do we even remember when our parents left.

We're both shitting bricks thinking we screwed each other, and the sheets are rather...rank, if you know what I mean.  If not, then you're too young for 4chan.

Advise?

The only sane thing to do would be to have sex with each other to see if it brings back any memories from what happened that night.