If Hilgya adopted the baby, the baby is hers. I don't make a distinction between biological and adoptive mothers.
Possibly, whatever undead monstrosity got Loki pregnant (and yes, I'm assuming Loki's the person that gestated Hel, because Loki will be Loki) left some psychological scars that justifies his "from now on, we say no to undead"?
(Yeah, yeah, straws are being grasped. I'm aware)
Grey Wolf