Yup, sounds like you got struck by the bug Ik.
Here's how it works, and how to avoid it. Simple version: To avoid this bug, do not travel with unmarried Sims. Not even Teens are safe.
This bug has everything to do with how locals in each WA location procreate. In order to make a Local baby, all that is required is for a male Local to greet a female local and BAM they're preggo. Except not really, because WA locals don't go through pregnancy, but within 9 days or however long it takes, the female Local will have a new child living in their house. I'm sure there is some sort of formula the game uses to determine when a child needs to be born, probably due to the population levels of the location. They will not have children with other locals who are already married with someone else, just with single locals and perhaps with teens (ew!).
The problem of course, is that when your Sim goes to China on vacation and the game has determined there needs to be more Locals, all that's required is for your Sim to say hello to someone of the opposite gender, and BAM in 9 days you'll have a full grown child that you never intended on having, living in some foreign location. No woohoo or pregnancy required!
This seems to be just the beginning of the bug. It must corrupt your game file to such an extent that it causes all kinds of other problems, like not being able to access the family tree, or loosing relative connections within your Sim's family, or worse case scenario your Sim or family may disappear or become playable but invisible.
I was VERY much hoping that this would all be fixed with Ambitions, but it obviously isn't, because people are still having these kinds of problems.
So the best way to avoid these issues is to limit your travel, and absolutely NEVER take an unmarried Sim on vacation!
