Basically when you save a family to the library and put them in a new town other than within the household the relationship panel is wiped clean, those friends they had before no longer exist to them, any family left behind no longer exist to them either and won't show up on the family tree anymore.
For example say you moved the Goth's to a new town, Cornelia will no longer have a sister (Agnes), Cornelia and Gunther will no longer have parents in the family tree as the graves of them do not exist, Mortimer will never have met Bella.
If you are not adverse to using Twallan's mods you might wish to look at Porter, if you packed the Goth's using that Agnes and the ghosts would be packed also, when you arrive at the new town and unpack them the family tree would be intact, now you could also pack up the Bachelor family with the Goth's and Mortimer would still be best friends with Bella. I think it might also put them into the jobs they had as well.
Overwatch and Errortrap would probably find any errors with imported families but I've honestly never had any from sims I've saved to the library.
Twallans sp is very adjustable, there are quite a few different settings for immigration you can play around with to get your town populated as you'd like it, and yes it will get sims married and have babies, find them jobs and actually advance them in their careers as well, it's very good.