It's time to replace sharing with taking turns. Author Heather Shumaker says:
"Sharing on demand" interrupts play, erodes parent-child trust, and teaches false generosity. Take turns instead. . . . Keeping a toy when another child wants it is not the mark of a selfish child, but simply a busy one. Protect your child's right to play and teach her to say, "I'm not done yet." Interrupting play — and instantly rewarding the other child — does not benefit either child. Waiting for a turn teaches great lessons in delayed gratification. Young children aren't ready to share; they are ready to take turns."