I agree I would put the babies together, perhaps when either babies sleeps through the night or at around 12 months of age. How would your step son feel about the idea of sharing with his younger brother?
i would put the babies together once they are sleeping through the night
i have a big age difference too.....my son is 7, daughter is almost 14 months and im preg with twin boys....we decided to put the twins in our room but we have no choice but to get a bigger house next year because there is no way we will all fit in a 3 bedroom
maybe move ur daughter with ur step son and create a nursery for the baby on the way. this way ud get the privacy u need and no1 will b bothered by any noise ..good luck!