It is, thanks! :)
Unfortunately, it looks like it's not just that Sim or that save file. I just tried to edit one of my other households and apparently, the mum wasn't related to any of her kids either. (Somehow, she's still related to her granddaughter, though. ?) I created this household weeks ago, and this is the first time I've noticed a problem.
In my case, the glitch only seems to affect female Sims who were created from the genetics of relatives who were created in a different household. I hope EA fixes this in the next patch.