We teach our daughter to share with us! Meals and playtime are great teaching opportunities --- we ask her to share with us, her yaya, even our pet dog! When she has playdates too with our friends' kids and her cousins, we always make it a point to teach her about sharing.
Sharing does not have to with siblings alone. With my unica hija, I only give her something only if she promises to share it with others, even with her yaya.