technically though incest only became illegal due to health implications of inbreeding. Prior to learning this, it has been a practice of many cultures. Soooo in a way, being "not blood related" does not remove the ick factor, but it does somehow make the reason for making incest illegal moot.
Though to be fair, it shouldn't matter for non omegaverse BL since there's no reproduction.
Its not incest. Incest Definition is blood related. Just becaus it feels wrong for you doesnt change the Definition. Father and son or adult and child is def something to discuss about but not when you grew up like "siblings" since none of you were adults or preying on another. They didnt choose to be like brothers it was chosen for Them.
ever heard of the term pseudo incest
Imagine If:
two people raised under the same roof at the age of 0, whatever the backstory for the adopted child, lives under the roof of loving parents and let's say.......... they look a 'little' alike, and basicallyyyy.... they didn't know one was adopted. They age up years later. Next thing you know, we get a little bit more than just platonic love and some goo goo ga ga shit happens in bed between the two
Now they don't know they're not blood related, so they're practically committing incest, but it's gonna be labeled pseudo-incest since they're not actually blood related; but the thing is, it's weirder because of the context
Just like how we can have an actual incest story where the two are separated and meet each other years later in the future only to find out they were dating blood-relations which happens irl much more frequently than the first one LMAO

I don't understand why almost every new BL contains incest. And they always try to justify it as "Not blood-related". If they grew up like brothers or father-son, it's very much the same like blood-related family