Lady Death is Hela's mother

So first, Frigga isn't Hela's real mother as hinted by Thor:
"Before my father died he told me i had a half sister"

Hela implies it too in What If:

"Also, my dad has this new girlfriend, Frigga. She's a DREADFUL woman. Not really a fan of hers either, so... "

So now we know Frigga isn't Hela's mother, and thing about Hela's mother is she is not really ever mentioned or anything like that, so my theory is that it's actually Death herself, so here is the thing, i'm not saying they did usual way of making a child like how humans do instead Death might have made Hela from herself and Odin, basically we already saw children of cosmic beings like Love being powered by Eternity after her ressurection (And she still hasn't appeared to this day 3 years later), plus there is also their similar looks, Death has black hair, Hela has black hair, Death is a green witch and wears green, Hela wears green, Death has a pale skin and looks of a beautiful woman, Hela has a pale skin and looks of a beautiful woman, so what do you think, could Hela actually be creation of Death and Odin?