Is it jealousy or am I doing something wrong?

My husband, almost-3 year old son (33 months), 4 month old daughter and I share a 900 square foot, 2 bedroom apartment. (Son has his own room, baby sleeps in our room.) For the last couple of weeks, my son has been waking 1 to 3 hours after bedtime and YELLING and SCREAMING until he falls asleep. I let it go for a bit, but then I wind up in his room, singing lullabies and trying to shush him back to sleep. We tried letting him sleep to his hearts content yesterday and forgoing a nap to get him to bed at a reasonable hour and alas! He has been awake for almost 4 hours. Between Hubby and myself, we've gone in there at least 8 times to try to get him back to sleep! He's woken the baby, his dad, and I just can't sleep when he's yelling, as much as I'd love to. We've tried singing, reasoning, ignoring....Now I'm willing to bet dollars to donuts he'll try to sleep until past his usual breakfast time! We've tried that before and it sets off a crazy terrible chain of events that just leads back to this same thing over and over again! I'm tired, I'm frustrated, and this really isn't fair to any of us! What do we do?!?!