Lol I don't get why some people called it "incest". They are not blood-related. It's legally not incest. If you think it is, you need to learn the definition a bit better.
Surely, the romantic tone is rather awkward because they grew up as a family and siblings. But under actual circumstances, it's completely okay for them to be together.
I always found it rather strange that the Western side's culture is always iffy about this issue even when they are not blood-related.
You surely don't have to like it, but calling it "incest" is technically by definition, a wrong term.