You're so lucky!
I am running three files and all three are on the 6th generation. Not ONE of them has had twins or triplets. I keep waiting and hoping but nothing so far.
SIXTH GENERATION?? You're the one who's lucky -- all my previous attempts have died out with no kids... I dream of making it to the third generation now - forget about sixth!
I just make sure they always have food, 1,000 or more, put them to bed before I go to bed, and then when I know I won't be able to check on them for 8 hours or more I pause the game.