Ok, it's quite a long story but I'll try to keep it as short as possible. I live with a girl that I've known for over 9 months now, and from the moment I met her we instantly clicked. When we met she told me about her boyfriend of a few years (I should probably mention that it's a long distance relationship, they only see each other once every couple of months), and naturally at that point I wasn't into her so it didn't bother me. But over time we seemed to grow closer and closer and I've grown to really like her. Anyway, she flirts with me all the time - I flirt back - and it's apparently obvious to everyone else that we're very close and that I like her. We've kissed a few times (just a peck - I wouldn't let it go further while she has a boyfriend) and she's even admitted while drunk that she's thought of having sex with me, and there have been a few times where she's leaned in for a proper kiss and pulled away at the very last minute.

Anyway, I don't really know how to act upon it. Her sister visited a couple of months ago and told me that I should make a move on her. Apparently her boyfriend feels threatened by me. Does she like me, or am I just the nice guy to fill the void left by her boyfriend? Should I tell her? I don't want to ruin the relationship we already have, but it's eating away at me.

Oh, and I know it's wrong to pursue someone who's taken, but I would can't really control my feelings and would never act upon them whilst they're still together. It's like the quote, "Meeting you was fate, becoming you're friend was choice, falling in love with you was out of my control."

I think she probably does really like you, but she's a little bit of a tease.

You probably need to quit with the flirting. You know it's wrong to pursue someone who's taken, so you need to back off totally.

Back off, stop the flirting and be a friend. I'm not sure why her sister would tell you to act on it, seems like a pretty unclassy move.

Since you realize it's wrong to pursue someone in a relationship, you should better mold your behavior around that notion. While she may be a "tease," I don't see you doing much to help the situation either. To be honest, I wouldn't want to get involved with someone who would do the things she does while having a boyfriend.

Sounds like she wants the best of both worlds. She has her long distance boyfriend who doesn't see her much and when he does, it's all fun. Then she has you to fill the rest of the time. If something doesn't go well with one, she can flip to the other. Now I ask, why would she want to change such an arrangement? She's got both of you hanging onto her anyway.

As for you, I suggest you put your foot down. Here's a guide for you: https://www.askmehelpdesk.com/dating/guidlines-what-do-do-if-person-like-already-relationship-463250.html

Make a decision and stick with it. Check out the thread to understand what decision I'm talking about.