I'm at the midway point of a legacy. The family has been traveling to Egypt about once a generation or so since Generation 3. For the most part, the issues had been minor until recently. However, one of my sims was married in Egypt and brought the spouse's family back to the home world, although when they loaded the relationship between the parents and their child had been broken. But they were townies, so it wasn't a serious issue. This time it is an issue.
One of the parents whose relationship with their son was broken before was still in my household when I sent the family back to Egypt. But after arriving, ErrorTrap started firing near-constant errors and resetting her whenever she tried to interact with other sims. I eventually decided to just send her home and I thought the game was stable enough after that. But it turns out that when I sent the home, all of their relationships had been completely wiped. None of the four children acknowledged their siblings as brother or sister, their father and mother weren't married, and the rest of the legacy sims had no connection to them either.
After reloading, I confirmed that the family relationships were actually breaking during the first day of my sims visit to Egypt. By midnight at the latest they would all lose their relationships.

I can't think of any solutions at the moment. I want this trip to happen. But I can't delete the .nhd file and create a fresh Egypt because I would lose the vacation home my sims have there, lose the relationships my sims have there, and then I'd have to delete the extra copies of all the sims I've moved to the main town. And no matter what configuration of sims I send (i.e, leaving behind the sims that were originally from Egypt, leaving behind all the sims that already traveled), the relationships always break.

I'm using ErrorTrap and Overwatch, but not Traveler or any other Nrass mods. I am curious as to whether or not Traveler might help with these glitches, but thought I'd come here for more general advice before trying anything. Any suggestions?

Traveler together with ErrorTrap and Overwatch that you already have to prevent the glitches and, for the few that might still sneak through, MasterController and MC Cheats to fix the broken family relationships.

I've just added Traveler, but unfortunately I'm having the exact same issue; the relationships still break come midnight.

Sometimes it can take a few sim days or a game reload before changes kick in. Did you remember to clear your script cache after installing Traveler?

Something else to consider:

If the script errors and family tree breakage persist beyond clearing the script cache, I would suggest zipping up some of those script logs and uploading them to us at NRaas so we can help interpret them. This is not typical travel behavior with the mods in play.
http://nraas.wikispaces.com/How+To+Upload
(free Wikispaces account and NRaas membership required to upload and open up a support thread)

Thank you for the suggestions. I didn't delete the script cache, but noticed the new options in the travel interface so assumed I was good to go. I will try sending them one more time after clearing the cache.