I already proved this wrong, that game itself makes it clear that they still have access to the black flames when we fight them, they explicitly drew their power from the Black Blade, and Maliketh sealed his weapon with his flesh after this. The Rune of Death was already in his word by the time he fought them, and what they used was a fragment of it.
Wrong.
The apostles, one said to serve Destined Death, are wielders of the god-slaying black flame. But after their defeat by Maliketh, the Black Blade, the source of their power was sealed away.
The source of their power wasn't sealed away until after Malekith defeated them, at which point he sealed it into his sword. Ranni later stole part of it and Malekith freaked out and stuck the rest into his body.
Because again, the game spells out that Marika had Malekith do exactly one thing, defeat the Gloam Eyed Goddess and take the Rune of Destined Death. Which happened well before Ranni or the others existed.
The only sibling Marika had was Maliketh, and even then as a half-sibling. There were no others except Radagon which was actually Marika. Either in a disguise, or as literally a split part of her being because magic.
We know there were other goddesses, and no indication of their parentage. They could easily be her sisters or relatives.