Does anyone know of Omegon's existence?

So, let's about Omegon, the problem child of the primarchs, the secretive twin brother of Alpharius. So I am wondering how many actually know of his existence, and as far as my research took me it seems that even the primarchs were unaware of Omegon's existence.

Ergo, I am asking you who actually know his existence. This is my list of confirmed and very likely people who know his existence

1. The Emperor (Obviously)
2. Alpharius (and by extension Omegon)
3. Erda (Cause you know she is their mother? But I guess there is a chance since this is Warhammer 40k after all)
4. Malcador (He is the emeperor's right hand man AND a pretty secretive guy who founded the assassinorium)
5. Valdor (The emperor's left hand man)

Bonus Question: Whose genes is the alpha legions's stable geneseed made of? Alpharius's? Omegon's? Maybe a mix of both?