We don't refer to it as incest though, dude. Let me give you the top 3 definitions for incest:Adapting words into a language is not the same as adapting a concept. In English we refer to it as incest, that's the beginning and end of it and no amount of stamping your feet because it's your fetish will change that.
n. Sexual relations between persons who are so closely related that their marriage is illegal or forbidden by custom.
n. The crime of sexual relations with a person defined by statute as too closely related.
n. The offense of cohabitation or sexual commerce between persons related within the degrees wherein marriage is prohibited by the law or established usage of a country.
Guess what? The only wiggle room for using the term incest in a non-blood related relationship is the 3rd definition. Only even that definition doesn't hold water in western countries as it's not a crime for step siblings or in-laws to have sex with each other or get married.
According to the English language, it's NOT INCEST by the working definition of the word.