<human>: Can you provide a brief rundown of the text in within 3 sentences?

I've always had a bit of a crush on my best friend's older sister. Recently single, I asked my friend (we'll call him steve) if it was okay with him for me to take her on a date and see what happens. Steve says sure, its fine, we're all adults. 

So we go out a few times and really hit it off, start hanging out and having sex pretty regularly. Turns out we're *great* together. Good news right? Not exactly...

A few months later Steve comes back into town for thanksgiving and, after hanging out with me and his sister - and seeing us together - he decided he's not at all comfortable with the situation and delivered us an ultimatum: end this, or consider our (lifelong) friendship over. 

In the month following thanksgiving, we have continued to see each other. He doesn't know about this and **I don't know what the fuck to do.**

Now, I love Steve. He is truly my best friend. But I'm also falling for his sister, and she is falling for me. I feel guilty going behind his back, and I know he'll find out eventually. But I also am angry with him for saying it was ok and then taking it back after I've developed feelings for her. I also think that he should be jumping at the chance for someone he knows and trusts to be with his sister, whom he loves very much.

Am I doing the wrong thing here? What do I do next? I'm pretty lost here.
<bot>: Best friend says it's ok to pursue his sister, later changes his mind - been seeing her behind his back anyway - don't know what to do.