Tell them this story: Once upon a time there were two lively children who liked to talk alot There sister asked them to be quiet, but they kept on talking so she cut off their tounges
Honestly I wanted my older sibling to shut up. But my younger sibling is quieter than me. Try to find something that amuses them. Maybe a toy,cartoons,movies to keep them occupied.
I'll say, "Let's play a game!" Then when they say, "Okay!" I'll say, "Let's see who can be the quietest! whoever stays quiet the longest gets chocolate! (or any other candy)" works like a charm.