I'm here with another birthday review!
I love how you characterize Peter here. I could see from the start that he really did care about his friends... but also why he might be targeted, and why he had the potential to turn, even if he didn't in this story. Gratitude can be such a tricky thing when it's not reciprocal, and I can see how he felt like he'd needed James and Sirius in a way that they hadn't needed him. That combined with jealousy - of what they were doing with their time, their looks, their wealth - can definitely turn to resentment, and it's pretty clear that it did. That doesn't mean that he would have volunteered to betray his friends, but it does make him more susceptible to it... and, while you take him down a different path (which I really like!), I can see so easily how things could have gone the other way.
(Also, while I know it wasn't the core of the story, I also found your characterization of Sirius to be really interesting - the academic aspect of his original plans goes counter to what (little) we know about Sirius as a teenager and what we see in his adulthood... but that feels right to me. We don't always foresee what's going to fit us well - when I was eighteen, I definitely had ideas about my future that wouldn't have been a good fit at all! - and it simultaneously hammers in that war doesn't just change your external circumstances. It has an effect on who you are inside, and there are some parts of you that just... get lost and can't really be found again, if that makes sense.)
Amazing job. <3