@Morotstomten DEFINITIONS OF INCEST:
•"Incest is the offense of having sexual relationships between family members or close relatives, INCLUDING children related by ADOPTION"
•"sexual relations between people CLASSED as being too closely related to marry each other"
•"Laws vary by state, but generally, a person commits incest if he marries or engages in sexual intercourse with a person he knows to be, either legitimately or illegitimately:
His ancestor or descendant by blood or adoption; or
His brother or sister of the whole or half-blood or by adoption; or
His stepchild or stepparent, while the marriage creating the relationship exists; or
His aunt, uncle, nephew or niece of the whole or half-blood."
I don't know why this is even a discussion, you can google this, BY DEFINITION adopted sibling involved with each other count as incest, this is the very definition of it.
You are mixing up inbreeding with incest, adopted sibling can have children but that does not count as inbreeding since that is exclusively a blood related thing.
Law variate from country to country and regions with in it, the law is about punishing those activities, there are countries where that is not illegal, others where it is, however we are not discussing the legally of that and if that would be punished by law, what I said is not that, I said that the adoption made the ML legally part of the family making this, incest by definition, since he is rightfully adopted.
I am not trying to condemn the ship, some people are into this, some don't, I respect taste, this is fiction after all, personally not my thing, but that's about it, it does not matter whatever I like it or not.
This is not an ambiguous scenario there the ML is LIKE family since they grew up together, and she sees him as his lil bro or something like that, (like the manga The Result of Being Reincarnated is Having a Master-Servant Relationship with the Yandere Love Interest), as I said, he was rightfully adopted by the family and not only that, he was also treated as part of it.
Whatever you may consider it morally right or not, it's subjective and up to you whatever you like it of feel comfortable with it, and that is okay, objectively it is incest, a tag for it is request again.