Ideas
This is how the family tree looks like after the newest update when viewing it from my gen 5 sim Mateo (his father, and his grandfather's fairy half-sister are missing, there are confusing connections that aren't real like some marriages or what looks like Mateo's sister but in reality is only a "Distant Relative" a.k.a second cousin)
This is how the family tree looks like from the legacy founder's point of view (actually clean, all info is correct)
All info is correct when viewed by literally anyone EXCEPT Mateo, and I can only suspect there's a case of overlapping portraits, which would explain what looks like a double-marriage or weird-looking lines.
I haven't saved after the update yet, this bugs me too much to play.
I have removed the localthumbcache file and moved the mods folder for the purpose of testing the game in it's vanilla state.