I wasn't sure if this was a bug or glitch or not, but it seems like it. Sorry if I'm wrong.

Basically, I just got Sims 4 a few days ago- I've yet to download anything but, like, sims and houses from the gallery. I have no CC.

I started off with two sims, siblings, they each got married to two different sims. Moved into two different houses. Both had their own sets of kids that grew up, and then their kids all got married and had babies. So, my family tree was being pretty productive, you know.
Well, I think it must have been just today, suddenly people on my family tree are gone. Like, okay, one of my original two sims died of old age? Suddenly she's not on the family tree at all. Her sister is, apparently, now an only child. And her husband (who is still alive) is no longer a brother in law to her sister.

The living sister's husband just died, and no he's not on the family tree either. Like, not even as a ghost, or one of those 'npc' squares that are just there for a stand in. It's as if no one but the members of the active household are in the family tree. I've tried clicking on each of them to see if the family tree would expand, and it didn't.

When I googled it, I think I saw that a few other people had this problem...like, ever since the latest patch or something?

Is there any way to fix this? >( Because it actually made me really mad.

Go to Manage Worlds > Household Management.
Do you see ghost Sims for the Sims you have played in household management?
Do you see a lot of unplayed Sims?

It's likely they have been removed by StoryProgression module. Research on the "culling issue" for more information.
The no-mod solution for this is you have to move Sims that you don't want deleted into existing lots.
If that's not good enough for you, you can try mods. There are quite a few flavors out there. Find one that suits your style and have fun with it.

People disappearing out of the family tree seems to be related to the games culling processes which kicks in when your game has over 180 sims including NPCs, townies & your own sims. Note: the dead are included in this number. I have the NoCulling mod installed to fix this problem http://modthesims.info/download.php?t=545615 . I recommend that you regularly use the games household manager to get rid of unwanted townies to prevent the game becoming slow from having too many Sims after adding the mod.