Longest running family?

Whoa, now that's an achievement. So far, the only kids I pick up with is Gen. 2; Aaron, Erik, Joey, and Brianna. For some reason, I just can't bring myself to follow Erik's kids. Oh, well.

Well, not by your criteria, but I have been playing my Cantrell family since 2005 when I first created Sim versions of them. 🙃

Since 2016. But "only" about 10ish gens in (depending on household/lineage, I am also a rotational generational player), because I play on very extended lifespans (each Sim lasts on average almost 2ish IRL years).
Still going. :) 
This is one of my households, which actually fuses two very old lineages. They are an affluent Brindleton Bay (=The Hamptons) natives, he is from a long line of doctors, she from a DSV "royalty" lineage (celebs/actors/musicians and such VIPs). Their kids are teens, the boy just turned 18 and graduated, about to attend uni. 

So far, four generations is tops for me. I play slowly and by rotation.

In all my years of playing Sims 1,2,3, and 4, I never played with aging on. I always had the same few households I would rotate between.

Only recently did I start playing generational style because I wanted to start writing my own SimLit. I'm on Gen 4 now and it's been really hard watching them age and die! I just turned the lifespan onto longest lol. 

I'm also a slow simmer but I can't use a large rotation as the reason. I just like playing my families at a slower pace. 

I have only one save and it includes five families that I've been playing since 2018.  Only four generations. And generation one, parents, was created in CAS long after generation two, Julianna and Edward. So technically I've only played three generations.

In addition to my five families I have ten families consisting of extended family members. I don't play these families but they are included in family activities, social events, clubs, sleepovers, etc.

Meet My Sims

Not that many generations - mostly 3. But many of my families have been in my save ever since the TS4 release. I'm just a slow simmer, because of my big rotation.

Going pretty good. And to answer your question, yes. My OCD (undiagnosed at the time, but now I know why I was doing it 😅) meant I was constantly saving them the my library so I’ve been able to import almost everyone. Except for cousins not really bothering with them. I’ve actually been sharing the process in the creative corner here on the forums! You could probably find it in my profile if you want to follow along!

As for friendships, I’m not really moving them, if there is a very special and/or important one for the story I’ll move them in otherwise eh, I just don’t want to add to much lag. And this generation I’m on honestly hasn’t made many friends 😅, but that works out cause my heir has 9 kids and is planning to have more…

Longest I have got to is 9 generations before my game got so slow I had to start a new one 😔

41 generations!!  Moving the whole legacy into a fresh save, wow!  How's it going?  I'm assuming you are uploading households to Gallery then placing them in the new save.  What happens to their previous friendships in the new save?  It starts new family trees, right?