D.H.
Let him know you would like to let his sister have his crib, because she needs a bed and would feel better in his. See if you can use that type of talk to get him to be a big boy. He may just be feeling a little left out right now, which is normal so going back a bit is getting attention and he's helping too. So, just make sure you let him know how proud you are of him and what a great big boy he's getting to be.....how he is going to be helping take care of his sister and how much she will eventually love that.........He'll be fine, just keep talking to him. Maybe they could nap together on the floor or something so that he would feel better about sharing and giving up his crib.
Good luck.