I think a nicer twist to this is that Forkle all but FORCED Elwin to stay quiet. Because Elwin said in Legacy(??) that Sophie deserved to know who her bio parents are. Even if their relationship did take a hit, it might help that Elwin WANTED to tell her.
That said, I'm about 90% sure it isn't Elwin tho.
If Elwin ends up being Sophie’s bio dad (I don’t think he is) it would probably cause like a major rift between them, if that makes sense. Sophie sees him as a father figure and he’s been her doctor for years of her life, in which most of her visits were fatal or just really bad.
I understand that if he was her dad, he wouldn’t be able to reveal himself but regardless it would be painful for Sophie. This is how it is between Sophie and Oralie, when she starts to resent Oralie (I think I’ve forgotten the specifics) once learning she was lied to for years by someone she trusts.
She would most likely have the same reaction if Elwin was her bio dad. And Elwin and Sophie were closer than Sophie and Oralie (makes sense he saw her in almost all of her near death experiences).
In my opinion this would also result in Sophie not going to Elwin’s office when she needs to, just to avoid meeting him. This would make her near death experiences WAY more serious and may end up with a disability or death if not treated (I think, I’m not a doctor obviously)