The incest-but-not-incest plot here boils down to something like a "siblings not related by blood" romance plot. But we magically know that the older brother did really think of himself as a brother (because magic lie detector) so it's a bit hard to countenance. To say nothing of the age gap. He also shows every sign of being... just a little bit of a creep, which is great for dramatic tension with the never-tells-lies bit thrown in, but leaves the eventual romance feeling even more suspect. At the end it's kind of like, "well, at least they're happy... I guess...?"
But yuck.
Like, for real human beings in this situation, I hope I'd be a bit more sympathetic here, them having been through a lot, but this is obviously a fully-engineered incest-fantasy setup for drama and fanservice's sake, so I'm not inclined to be so charitable.