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The game does not allow to change the family relationship status for preexisting sims. Such as adding a child to a family, - the child already previously living in the world - then going into Cas to make one or more adults the biological parents of that child. It's a no go, as the child already has their biological parentage set up. Even if you don't see that genetic parentage displayed in the family tree. All sims spawned in game that do not come directly from families via birth, are made from a genetic mix of existing sims, so the random genealogy could be the likes of Mortimer Goth mixed with Liza Pancakes. Hence why it does not display in the family tree, otherwise could get mighty confusing. Forget the possible incestuous ramifications! Maxis certainly do.
The best workaround to rectify this, is to first save that child to your Library, where their genetic parental genealogy will be wiped, then add the saved version of the child to family via Cas, where you can now change the family status for the child to be biological offspring of those preexisting sims. Yes, technically speaking it is a brand new sim. But they'll keep all traits and skills they had previously. Then of course you'll have to delete the original sim. Not unless you want the doppelganger also living in the world.
"Become a government informer. Betray your family and friends. Fabulous prizes to be won!" Red Dwarf - Back to Reality.