villagers just vanish

okay so i'm on my brothers vanilla LAN server that we made two days ago and we made it pretty far and a the start we found a village and i trapped the villager now like 2 hours later i build a 17x17 house i then put the two villagers in it trade with them so they breed and about an hour later i had 13 now my brother left minecraft running so i can play the world while he played with his friend so he was afk i made absolutely no way for mobs to spawn or for them to get out now i had to go eat so i was afk for 10 minutes and when i came back the were all gone and i just thought that my brother killed them but he swears he didn't so then i find three villager zombies and i cured them after ward i led them in the same pen and breed them soon after i got 5 villagers i went to take a shower and i was on top of the pen and 15 minutes later i come back and there gone again this time my brother was a the store and there no way a zombie could get in to them or spawn in there i checked the lighting of the room with f3 so i wish to know how to prevent this from happening again. please help