Babies all sleep through at different rates - my girls were all in Reception before they slept fully through the night - but don;t let that scare you!
Make sure baby is fed and winded, a warm bath may help to settle her, and no stimulation at all for the lasthour before she goes down.
Just reasure her if she stirs, be in the room, and speak very softy to her, and lean over her cot so she can smell you, and be assured you are there.
Don't worry about your son, he will soon adapt to the disturbance, children always do - so be patient, she will get there.