>>13304530>>Elly's not Fei's mother per se, rather the Zohar's attempt to replicate her to endear her to little Fei so they can procreate later. It's only incestuous in the concept.
Not exactly, the Zohar created Elly for Fei and even if at start it was to be Fei "mother" Elly started to be Fei "lover".
In case anyone is looking for blood relations, Fei is the only survivor since Elly was, like the rest, a creation of "God" but was a creation by the Zohar for Fei.
Considering Xenogears nature its likely Elly was to show how men are always looking for "mother" in women.