Slept With Friend's Sister

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TL;DR: Slept with good friend's sister, haven't talked to him since and now is the time. What do I say?

During November last year, I had a fling with one of my best friend's sisters. She came onto me one night and I just went with it, this turned into a thing that lasted until mid December. We had sex once and messed around three or four times. Unfortunately, she pretty quickly (and unexpectedly, due to the amount and variety of previous relations she's had) caught feelings and refused to take no for an answer once my sanity caught up with me. As a result, when I finally cut things with no minced words, she got pretty upset and her brother found out as they live together and he was very pissed about the whole thing.

Fast forward to now, I haven't talked to him since then out of shame/guilt, however there's a large friend get together tomorrow and I figure now is the time to at least figure out where he's standing before the event. How on earth do I open that conversation though? I have no idea how to approach him about it, even via text. As an added layer of "oh fuck I messed up", she's 17 and I'm 22 which is clearly pretty bad. I made a concession due to how experienced (much more than I am) she is though, that's hardly a point I can bring up to the brother however.