I realise that the thread is a bit old, but it's very related so I'll reply rather than starting a new.
The exact same thing has happened to me several times! Quite some time has passed since you lot started to experience this problem so you would guess that some patch would have fixed this by now?
I've had the same problem in Egypt with a random child, and it has also happened in France. I don't think you need to have any relation with the person that you randomly happen to be married to or have a child with, but it seems they are usually linked to an adventure.
In my current game I made my sim go to France and complete a lot of adventures only to be able to buy a vacation home. When I returned the second time, it seems that she is somehow married to a deceased guy (Remy Dutiel) and they have a child together who is a teen. The teen girl lives with her aunt in what used to be Remy Dutiel's home. My sim now has the same last name, Dutiel, and the girl and Remy's family tree has been joined to mine. Unfortunately I had just saved so it was impossible to go back, so I figured I would just add the teen to my family and it would be a weird 'twist' to the storyline. I'm only in the 2nd generation so it's not throwing away a lot if I would simply start over, however it's very frustrating to do so, so I decided not to and just go along with the weird story instead.
However -- the most irritating part is this: when they (mother and teen) return home, all of the previous relatives are now unrelated! They are now only affiliated with the French people on her daughter's side and not her own back in her hometown. She used to have a sister and father with whom she lived. This just bums me out totally because I love playing for the family tree and the story. It completely throws off the whole thing. Now her old dad is just an "old friend" and even gets the "attractive sim" moodlet because they share some traits. It icks me out!
Is there some way to repair the broken family tree? I would love it if she wouldn't be attracted to her father... it's quite creepy.
I've been searching all over, mostly by googling the trouble, but haven't found any solution or even a similar problem. If the family tree is repaired I don't care too much about the extra unexpected child.