Anyway! LullabyKnell was reacting to an apparent trend in CC fanfic to make Delphi not actually Voldemort's kid, because someone polyjuiced into him to have sex with Bellatrix, and whoever that was is really Delphi's father. Delphi notes that:
if Delphi was conceived after Bellatrix’s breakout from Azkaban and born before the Battle of Hogwarts, then that means the biological component from Voldemort would have been from his second body. The resurrected-in-the-graveyard one that we only know to be made of a) MAGIC, b) Tom Riddle Senior, b) Peter Pettigrew, and c) Harry Potter. So is Delphi even biologically the original Tom Riddle Junior’s kid? Wasn’t that body destroyed?Zie then goes on to speculate as to what would happen if, somewhere along the line, Harry and co. figured this out and realized that she was biologically his daughter?
My thoughts turned in a different direction.
The bones of that resurrection potion of Voldemort's came from his despised Muggle father. And blood is made in bones, so while Harry's blood was a necessary part of the potion and no doubt provided a symbolic link, Voldemort's actual blood, once he'd been alive again for a while, all came from his bones ... which came from his father. He is no longer a halfblood; on a biological level, he is now a muggle-born. He has no blood connection with the Gaunts, and hence Salazar Slytherin, any longer. (If he had realized the implications, he would have found a different way.) There are some parts of him that come from Pettigrew, but they were mostly fuel and scaffolding. The genetic bits came from Tom Riddle, Sr.
So he and Bellatrix have a kid. Except that what neither of them realize is that, biologically, Bellatrix and Tom Riddle Sr have a kid. (Really, Voldemort should consider himself fortunate that dear Bella never figured it out. There is not much that could break Bella's loyalty to him, but causing her to bear a half-blood bastard would do it.)
But this is Bella and Voldie, here. Not really parent-of-the-year material, and anyway, while Voldemort wanted an heir to show off, he doesn't care about them until they get old enough to get molded into what he wants. Bella, meanwhile, even if she wanted to be a mother to the kid, is in a bit of a delicate spot with her husband, who is slightly less devoted to Voldie than she is. So the kid gets dumped with a house elf for a nanny, while Bella and Voldemort are off fighting. There'll be plenty of time later, right, once they've killed all those dirty mudbloods and blood traitors.
So, in the aftermath of the battle, there's a lot of mopping up to do--death eaters to chase down, DE strongholds to clear out, people to put on trial, etc. The Trio pitch in, but Shacklebolt gives them the assignments least likely to lead to violence. (He knows they can fight. But he also knows they're kids and, now that the prophecy is over, they shouldn't have to.) So that's how they're the ones clearing a DE safehouse when they come across a toddler and a house elf.
The house elf knows its master and mistress are dead, and its loyalties are free. The house elf isn't a Black elf, so he has no loyalty to Bella that isn't compelled by its binding to her, and Voldemort's never had a house to have house elves for. The elf's true masters were killed by Death Eaters, so he is perfectly willing--ecstatic, even!--to hand over his charge to Harry Potter, thank him for freeing him, and go off to see if he can find any relatives of his true masters. Before he goes, he tells them everything he knows.
Harry and Ron, hearing that this kid's parents are Bellatrix Black and Voldemort himself, get grossed out. Hermione, who knows a bit more about genetics and inheritance than the boys, gets a weird look on her face and starts doing tests. (She has quite a strong interest in genetics and inheritance. She has a whole lovely lecture pointing out just how WRONG and STUPID the whole pureblood notion of inheritance and blood purity is, to be used in response to a slur about mudbloods, which she's never gotten to actually use because what's the point in lecturing someone who won't understand how WRONG they are even after you tell them?)
The thing she's most worried about is that the baby might be Harry's, genetically. That possibility would be the hardest to deal with, because of the personal implications. But it is quickly ruled out.
A Pettigrew connection might be the best-case scenario, and it's easy to find his body, get a sample, and compare it. That, too, is ruled out.
Hermione and Ron go back to the graveyard where Cedric was murdered without Harry. Someone needs to watch little Delly, after all. They get what they need. It's a match. Delly is, biologically, the daughter of the muggle Tom Rilddle, Sr, and thus the half-sister of Voldemort.
It's almost a good thing there are so many war orphans; Delly Riddle is very easy to slip in among them. The story they give is completely true. It's just missing a few details. Delly Riddle, they say, is a muggleborn whose parents were killed by Death Eaters. Delly Riddle, they say, was found by a witch and given over to a house elf to raise and keep safe. That witch, they say, died in the war. Delly Riddle, they say, has no living muggle relatives, so now she needs a new wizarding foster or adoptive family.
No eyebrows are raised. It's a believable story, and more than they know about a couple of other very young war orphans.
(The irony is, a few years later, that one of those other war orphans with a mysterious past gets fingered as a possible Heir of Voldemort. Nobody quite knows how the rumor starts, but once it does, people get hysterical. The Trio watch with helpless fury, seeing what it does to that poor kid. They do their best to dispel the rumors, frustrated that they can't prove how they know. After all, they can't say "We know Melody Legato isn't Voldemort's kid because Delly Riddle is.")