||The worst player is Marina's sister. Draw a diagram,
putting Marina's sister and son directly across
the net from each other. It doesn't matter whether
they're on the left or the right side of the diagram.
Now, try putting Marina's daughter on the same
side as the sister. The daughter will be diagonally
across from the son (the best player's sibling),
meaning that the best player is the daughter.
Since the daughter (the best player) is on the
same side as the sister, the sister must be the
worst player. Try putting Marina's daughter next
to the son. The Marina will be the best player,
but the sister will still be the worst.