Hey there, I also have a boy (who will be six next week), a three year old and one year old girls who share a room. My first suggestion would be to just put her down, see what she does. It might be difficult, especially since she might be used to the swing, but it's worth a shot. Also, how deep of a sleeper is your oldest girl? Maybe if the little one cries a little, it might not be enough to wake the older girl. My kids have shared rooms at one point or another (my son and middle daughter shared a room for a little while, now he has his own room and the girls share a room) and it's a temptation to think about them waking each other up, but so far it's worked out well. My son is a deep sleeper (I mean DEEP, never woke up because his sister was crying in the crib next to him!), and my middle daughter is at an age now that if the baby cries (which thankfully, doesn't happen often anymore!!!)and wakes my older daughter up, I can just talk to her and say "It's okay mama, baby's having trouble, just go back to sleep". Anyway, sorry to blab, hope that helps! Eventually, your youngest won't have that middle of the night feeding and she'll be in her own room, so keep the faith!