Sharing does not come nauturally to most children, but he needs to be taught to share - both by being positive to him when he is sharing and by denying him attention when he wont. If it was me, when he refuses to share, i would choose another toy and get involved in playing with the little girl - make it sound really fun. He can join in if he wants, but only if he brings the disputed toy with him and shares it with everybody. If he doesn't bring the toy to share, do not allow him to join the fun - he can go and play on his own with it if he wants.
He also might find it easier to share it with you first and then you share it with his sister - but persist in getting him to share directly.
If you must give him sweets, I would use them as areward for sharing and wouldn't give him any otherwise.