Moving in with sibling advice please!!

[bumpsy]

Hi I am expecting my third baby in March and I need to move my ds age 15mths in with his sister age 5 they are both excellent sleepers so I thought that would cause the least amount of disruption.The only trouble is ds is so used to sleeping on his own in his very small dark room I wondered if I it is likely to upset his excellent sleeping record or not and if anyone had had similiar experiences.
He also has a two hour daytime sleep in his cot in his room.I know the new baby will be in with us for a little while but I thought if I did it now he is less likely to feel pushed out of his own room by the baby.Any thoughts?