Mankar's children are not aspects of himself like Divayth Fyr's daughters, or at least they weren't originally anything more than normal children. Ruma Camoran rebelled against the Mythic Dawn, was killed by her father, and, through his intimate knowledge of the Razor, was brought back to serve him.
Oblivion is a dimension whose logics are governed by the reigning Prince of a given realm. Mehrunes Dagon encompasses destruction and violent revolution, and his slave/pawn/hierophant Mankar Camoran earned a small dominion of his own through his submission to the Razor. In his Commentaries, Mankar describes his Dagonite beliefs regarding parenthood, childhood, birth, and reproduction at length, and while they are as nonsensical to you and me as that MK quote you just posted, in the laws of Paradise, and the broader Deadlands, they make perfect sense, or at least enough sense that they work.
Still, I can take a stab at this.
Mankar is talking about her re-birth, here. Keep in mind the whole initial rebellion-and-dying part. She was birthed by herself (in the identical image of her former self) and her father Mankar Camoran was the father - basically, what he's saying is the concept of Ruma Camoran and Mankar Camoran gave birth to Ruma Camoran. The second sentence is more resistant to interpretation, and we sort of have to take it on its face - she had a hand in the creation of her brother, but not as his conventional biological mother. I just assume that Mankar created him the same way he re-created her, with her help, and that it wasn't really incistuous.