I'm sorry to hear about your Sims, that is extremely annoying when things like that happen. I read this somewhere online, I thought, if you haven't tried it, then what's to loose?
Ok so they had a similar problem... their Sims were just vanishing. they tried resetSim and all those things and nothing worked, so what they did was load up the game (the Sims would still be glitched) and as soon as you entered the game, go into edit town mode.
Save a copy of your family and household to the clip board and bulldoze the original household. Then, move the copy from your clipboard back into the empty lot and start playing again.
I hope this helped!
Happy Simulating!
Foxx