A.L.
B.:
It would seem that the real problem (in the incident that you described) was the scratching, not the sharing. Children that age do not understand the concept of sharing. Yes, you can start teaching them to share (letting her cousin play with the toy for a few minutes, giving it to her saying "we share with our friends", and then giving it back to him....) however, they will not "get it" until closer to two years. The scratching however, can lead to bigger problems. You might want to say something like, "we do not hurt our friends", and put her in a time-out spot.
A. L