I have this weird scenario in my head where he keeps her as mistress and has at least one child with her, but engineers things so that said children have no idea about their parentage, in the interest of having them work their own way up in the world the way he did. Then he picks the most successful one to claim their heritage only after they come of age.
That sounds like it could be the plot of some novel about political intrigue.
But in reality, the Nazis had no qualms at all about Nepotism, Göring and Bormann both placed siblings in high positions, Magda Goebbels fancied herself as a First Lady, and Hitler let his nephews get away with anything.
For instance, his reported favorite nephew was Heinz Hitler, he was a teenager when the Nazis took over and there is at least one reported incident where he used his name to get away from police trouble. Hitler also offered an exchange of prisoners with Stalin when Heinz was captured in Moscow, offering to return Stalin's son Yakov, the famous incident where Stalin refused to help out his captive son. Heinz ended up dying in captivity.
He also left pensions and money for his half-sisters, Angela and Paula, despite considering them stupid and disapproving the later's remarriage. He also moved them away from the approaching Soviet Front through Bormann in 1945.
And let's not forget William, the one who later served the US navy. William Hitler was all about landing fancy jobs through Hitler, such as one in the Reichsbank, and constantly blackmailed him about spreading embarrassing family stories if he didn't get more money.
Point here is that Hitler benefitted all his relatives through Nepotism, some more directly than others, I hardly think he would do the same to an actual child of his'.