It's hilarious that people say 'this is incest!' when non-blood related adults get together... 'incest' has to do with bloodlines, with screwing up genetics. Not with non-related people raised under an umbrella name(heck, he wasn't even raised with her for most of their childhood given the whole 'ship him off to boarding school'). You all might as well cry that orphans raised in the same orphanages are 'siblings' in that case and decry and moan about that, while you're at it.
If two non-related adults falling for each other bothers you, as it appears to be given the cover art, that is honestly not the author's problem.