Anyone know how to

Get your March baby to share? I know it’s the age. But my 18 month old will take everything and anything away from his little sister. She is 5 months and loves him but he will not allow her to have a toy in her hand without claiming “mine.” And telling her “no sister”. We tell him again and again that it’s sisters turn and that he has to let her play while sister has it. She can’t even lay on the floor mat without him saying mine and attempting to get her off.