I think that Gethen is Sophie's dad because they are both blonde and they are both extremely good telepaths.

Gethen must have joined the Neverseen right after Sophie was created, like Gisela.

Forkle did not want Sophie in his head because he thought that Gethen might tell Sophie that he is her dad.

Both her parents have a good reason to be kept secret so Forkle tried his best to stop Sophie from finding out.

Edit:

teehee