SUBREDDIT: r/AskReddit
TITLE: (need help fast) dating an ex's sister
POST: I was seeing a girl a month ago, we only made out at the time, and we knew it wasn't going to work out so we split up.

But when going to her house I also got to know her sister, we started texting and went on a date.
Yesterday I went to her house (the other girl wasn't there) and we had sex.
Today she sends me a text asking if I think she's my backup plan or something, she found out I made out with her sister, which I thought she already knew
Now she's coming over to talk about it, but I have no clue how to tell her that I really like her and she isn't some kind of backup plan, any help?
TL;DR: 
Dated girl, didn't work out, started dating sister, she found out I dated her sister first, she's mad and coming over, what do?