I skim read this first and will go back and read it more thoroughly later. I wasn't quite sure how you planned on ending this since usually you alternate POV's so that if last week was Teddy's POV we wouldn't see what happened with Teddy/Andrew in this chapter. But I like how you worked them into the story.
I'm probably going to be the only one who is going to say I'm disappointed with how the story ended up. Somehow I'd always hoped that Remus and Teddy wouldn't go through with it and would remain some what faithful to their partners considering everything they'd already done and especially after proclamations of love. To me, their relationships will have deep secrets and lies and I think that is oh so very wrong. Andrew will continue to think that Ted will have been his first since that's what Ted told him, but it's a big lie even if he never finds out. And I believe it would hurt him terribly if he did.
There have been other incest stories between siblings that somehow didn't bother me because there were no attachments with other partners with the participants. But because other people were drawn into Remus/Teddy's circle, then it's a web of lies and deceit and that really does bothers me. I really enjoyed the story until the last two chapters, especially the chapters with their partners and the chapter where Teddy seduced Remus.
Unfortunately, I'm disappointed with the way the story ended and it hit even futher home when Sam and Remus were married, how unfaithful physically and emotionally Remus had been to Sam and yet he still lives this lie that he had never been. And Andrew here - well, we'll try it the other way 'round sometime and you'll see. So innocent and so deceived by the person he loves. That is so very wrong. Tell me how many people you know that would be OK with just the affair if it was a one off? No wonder Remus and Ted have to keep it secret, and not just because of the incest.
Their happiness with what they did and with their partners at the end doesn't hide the secrets and lies. It's a good story with an unfortunate ending that in a way makes me feel like I've been betrayed and makes me very sad. Incest was one thing, it's the affair that was so wrong no matter if it was called teaching or not. At the end it wasn't, not really and that hurts.