Then u and hubby sleep in same room as big b and the servant stay with little b. When little b does not need night feeding every few hours, big b can move back to the kids' room with the servant (in about 4 months after little b's birth)
How old is big son? Old enough to sleep on top of a bunk bed (2 level)? then buy/use a bunk bed+ baby cot in the 3rd room.