S.B.
No, don't move him into his sister's room. Though her presence will be a comfort to him, he does not need to learn that he has to have someone in their to sleep with him to go to sleep.
Just keep doing what you are doing. When he gets out of the bed, hold his hand and take him back to his room. No chit chat, just tell him it is time for sleep and you love him. Kiss him, cover him up, and leave.
You should also do that when he is going to sleep at night. Do not allow him to fall asleep in your presence. This will be a hard thing to undo later in life.
Well I just read your post and saw you caved in and put his bed in his sister's room. I am seriously thinking this was a terrible mistake. One that will take a long time to undo.