Adrienne, my two cents here. You MUST tell her Mother. Mom needs to know. This could be just the start, or could be something she is already into. My gosh, how much did she drink? She was actually "out" on your stoop?
I have a 17 year old daughter, if someone kept this from me, I would not be a happy camper.
One other question......do you allow her to have friends over while she sits for you? If she did this all alone, in my opinion, that is more disturbing. I think we can all agree, getting drunk at some point with friends was a sort of "right of passage" I must wonder if she let someone in your house while you were gone.
Sorry to ramble so, but in my opinion, Mom should have been called right away. I know that can't be changed now, but please inform Mom.
This is not a fun place for you to be. Your losing your sitter and you may end up in the middle of a family squabble. Bottom line......your kids safety comes first, and she is not someone you want caring for them ever again.
Sorry so preachy........but it's early and I'm still on my soapbox
Good luck to you, not an easy task before you.