L.D.
My daughter has 3 boys in the same room because of space constraints. They are 7,5,and 1.
When the baby was born, he would wake them up once in a while, but it didn't take long for them to sleep right through his crying.
I believe it is a matter of them getting used to being in the same room. And, they all go to bed at the same time, which lets mom and dad have some time together.
You didn't say when bedtime was, but at my daughter's house, 8:00-8:30 is bedtime. that way mom and dad have time to talk, watch tv or do something else in the evening before bed.
H. that helps.