I feel it depends on your toddler whether he is easily awaken by noise. If you allow them to sleep together, I think in the beginning, it would need some getting used to. Maybe you can let them sleep together for nap times? Then you can "test" to see if your toddler is able to handle his sister crying plus let him get used to the fact that his sister is sharing a room with him.
My boy shared a room once with a toddler at a friend's place. Their toddler couldn't get used to my son crying and would startle. I think it's coz he is not used to having someone else in the room, so I believe it would be the same with your son. But still, I'm sure he can be conditioned.
Just my 2 cents worth.