I sent triplets to three different boarding schools for my 4x4 game. They are in gen 3, so the game has been going on for quite a while in sims time.
Only the eldest remains on the relationship panel. It's very strange. When I went to the "move" screen via the phone, all three show up, but there is no way for me to pull the younger two out of school or contact them. I played through when they should've aged up to see if they'd come home on their own, but they didn't in the four day lifespan for children, even the one in the panel. Not sure if this is bugged beyond repair or if there is a reason for it.