I think give him the choice. Ask if he wants to sleep in his big boy bed & really encourage it. Then if after a week or so of him sleeping in there, maybe ask if its ok if his sister sleeps in the baby bed now. Explain to him that she's getting too big for her bed.
I think let him lead the way, or think he is & things should be ok.
I agree with the gate to the lounge room too. I've never had gates or closed doors to the bedrooms. If my babies need me they come to me. Just remember to tell him where you are if he needs you. That you'll be in bed, not in the kitchen etc.
Good luck