It's either incest or it's not.
They're either biologically related and it's incest or they are not and it's fine. There's no "pseudo incest".
I mean it would be a bit weirder if they were together since they were born as siblings in the same family, then they would most likely have a lot of brother-sister feelings for each other.
Here they seem to have only come together into one family after they were already "self-aware" and like 8+ to 10+ in age?
So they would have about as much brother & sister feelings for each other as you and a random kid had in 2nd or 4th primary school --> basically 0.
I mean the only difference between them and a childhood bff is that their parents are married by law. It's literally just an artifical human made law that society accepted.
If their parents weren't married and just lovers it could be a totally normal double romance story most people would probably even find "cute". The big problem people have is most likely just because their parents are already married, by law, and thus they count as siblings, by law. If they came together before their parents married, then most people woudn't even bat an eye, I'm sure.