They came out of the same woman, they look similar (at least in their human appearance), they grew up together as siblings and they recognise each other as siblings. Regardless of whether they're the same "species" or not, I think that's more than enough reason to qualify this as incest, at least in my book.
Ultimately, I think it boils down to what readers consider to be incest or what exactly about incest makes it problematic. Is it the being raised as siblings part? Would a story about two real siblings who never met until they were adults and never found out they were related be less disturbing? Is it the actually being physically related part and the resulting biological defects that are the real issue? Is it both? Because, yeah, regardless if they're actually related or not, the "twins" do consider themselves to be siblings, so the shame involved in be attracted to someone you consider family is still there. I think the fact that they're not blood-related is there as a comfort to readers that their relationship "isn't THAT bad" but the incest-y elements are still there, and I think their relationship is still supposed to makes us uncomfortable, if slightly less uncomfortable than biological incest.
The FL and ML know each other as siblings, but they have romantic feelings for each other? I think that is wrong.
The ML is not blood-related? Is it still weird for step-siblings or adoptive siblings to have romantic feelings for each other? I think so.