I'm 22, and I have an 18 year old sister. My sister has

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I'm 22, and I have an 18 year old sister. My sister has a good friend of the same age that I've known ever since they were both about 13.

I consider her friend to be a close friend of mine as well, we have always got along, been on good terms, I chat with her on text from time to time, etc

We've hung out without my sister a couple times, in a lot of ways she's like another little sis.

Over the last couple years or so, ever since they were about 16, my sisters friend (let's call her "meme"), started looking at me different, and talking to me differently. I see a glimmer in her eyes that I only see around people I've dated.

She giggle or laughs sometimes even if what I said wasn't funny, and she touches my arm and things like that. She hugs me more often and for longer.

I thought I was being paranoid and weird, so I brushed this off until a couple months ago when meme was going through a breakup. We were hanging, and She kissed me out of nowhere, on the lips, and when I asked why she said she "needed it"

meme kept doing affectionate things towards me until I noticed that I was getting very sexually attracted to her based on all the stuff she was torturing me with.

Everytime I see her now I'm hard as a rock the entire time, and it takes all my willpower not to make a move and take her to bed and make love to her when my sister isn't here.

She knows it too, she grins and winks and smirks when she sees how uncomfortable I am.

I'm seriously considering doing it, but I love her like a sister and I don't want to ruin our so far platonic relationship.

What do I do? Does anyone have any experience dealing with complicated relationships like this?