DD slept through ANYTHING - they weren't sharing as babies, but we had a really small house, and later when they did share he rarely woke her. You can likely do separate bedtimes by involving DD1 in putting her sister to sleep, but DD2 might wake up when DD! comes to bed, it depends on how she sleeps. But when DD2 is three....uhuh nope no way. If older sis gets to stay up later, good luck getting the littlest little to go to bed early! We can't manage it even now, and they are back in separate rooms!