kids learning to share

My sister's 2 year-old has trouble sharing toys in general with my 1 year-old. If my child goes to swing, she wants too, if grandmother has her in the arm, she runs over demands it.

No one seems to tell this child "not right now" or "you can't have this"

I'm more outspoken and nicely say a along the lines ... you have to share because it shows you care. Grandma can't right now, when she's done she'll take you. When my sister is there I let her take over and there's a no but no consequences. I'm at the point where I should avoid the cousins bringing together since no one can't discipline.

What do you guys think?