I know my friend for 10 years now and his sister is constantly showing IOIs in the past few months. She is touching me when we're out with the group of friends (poking my stomach when I'm making fun of her, touching my arm while talking and so on) and is always trying to hold conversations with me. Additionally she messages me every few weeks or even days. I like the playful teasing with her and find her attractive and interesting, but there are some issues with this (obviously):

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  • She's in a LTR
  • She's the sister of my friend

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The LTR is going bad. My friend told me about this multiple times and you can really see the issues/problems they're having in their relationship. They are a couple for 4 years+ now and living together for some time. He isn't using any form of dread and always tries to please her: typical blue pill/nice guy. I experienced the monkey branching/hypergamy multiple times in both directions so I am very aware of what's going on.

In contrast to this I'm always treating her like my little sister and flirt with other women in front of her which makes her understandably more interested in me.

My friend told me that he would love to see me being a "part of his family" and suspects the LTR to end soon, so it's like a green light from him to step it up but I'm hesitating due to the above mentioned circumstances.

How the fuck do I handle this? Just keep up the playful banter with her but don't escalate until she loses interest to not shit where I eat? I think this would be the best solution to this (because I keep everything easy and don't take the risk of destroying my friendship) but there's always this desire to give this one a try. Additionally it would be hard explaining my friend that I'm fucking his sister and not straight going into a LTR with her as I don't want to do this again as I did in my blue pilled time.

Fuck, help me clearing my mind on this one and setting my brain up for giving up on this one.