I think there are many sides to this conversation. It's best to enjoy the writing
I agree with you on this, it was what I was thinking. The story was interesting even though I didn't like what Remus and Ted ended up cheating physically and emotionally on Sam and Andrew, not necessarily the incest.
I believe lying like that and keeping it a secret has a detrimental effect on a relationship. Remus may be able to keep the secret, but Ted has already slipped a couple times, what happened may come out. Ted has also already lied and manipulated Andrew to have sex at Remus's house for personal reasons. What would Andrew have said if he knew the real reason? That doesn't seem healthy. Maybe Andrew would have said OK if he did know, but he wasn't given the honest choice.
Lying and I do call it cheating, since Remus and Ted were in love with Sam and Andrew at that time and Andrew went through a lot to be with Ted. He thought he would be having Ted to himself and he's been completely lied to. I think that's much worse than Remus/Ted relationship, although I don't think most parents would have sex with their kids just because the kid as low confidence issues on sex. If there were no Sam and Andrew, then I don't believe this story would have bothered me as much.
Like I mentioned though, I think I'll probably be the only one who thinks that it's morally wrong to cheat either physically or emotionally with someone no matter how they are related, when that person already has a partner that loves them and you love back. Unless it's open and is agreed upon by both partners.