So it's most likely not going to happen in the fic I am writing for him, but imagine Mourningstar's delusions go so hard that he genuinely believes that you are his babygirl, and not in the adopted way. Like in order to help him heal more his brain decided to fabricate that everyone died that night except you. You were taken from him, but not by death. The angels must've taken you and then sent you back down once he proved he wouldn't lift a finger to help those filthy sinners ever again.
Even if you look completely different, even if you insist you don't remember this man he's so sure in his delusions. Of course you don't remember him you were just born when it happened that's why you look at him like he's a stranger, oh babygirl he's so sorry he missed so many important milestones for you. He wonders why the angels didn't tell you about your Daddy, but it's alright he'll tell you all about your family and about memories he has of you as a baby.
It all makes so much sense now! Angels wouldn't be so cruel as to murder his entire family. The reason he didn't make a grave for you was because he couldn't find your body, and a small part of him held hope that you were alive somewhere. The reason why Charlie's nursery only had the one crib was because him and Lillith had decided to co-sleep with you in their bed, yes, yes, it's all so clear! He must've been so deep in his grief he had momentarily forgotten, oh babygirl he's so sorry but he remembers everything now!
He will tell you all about the day you were born and how excited Charlie had been for a sibling. He'll tell you about how she used to play dress up with you and read you her favorite stories and how you'd giggle. He'd tell you about how he used to make you watch in wide eyed wonder whenever he'd use his magic around you.
What you aren't his second daughter and you came from another family? Oh honey, the angels must've kept you in a state of sleep and you dreamed the whole thing, shhh, don't get upset now, Daddy's here, and he'll help you remember the real truth about your past. Poor thing you must be so confused, don't cry Daddy's here now.
For those of you who have no idea who this unhinged man is allow me to introduce him to you. Essentially he is Lucifer if Lillith actually attempted to form a legitimate army to take down heaven due to the extermination and as a result was punished by the angel of death taking away his wife and child. He becomes more and more secluded and insane in his grief until he finds an Omega Little regressed, being chased by an exorcist (in my fic), crying out for their Daddy and he saves them and becomes immediately attached and regardless of pronouns or actual gender he calls them his babygirl or princess. He is unhinged and delusional but truly does love them.