When I turn on my upstairs faucet it overflows my downstairs sink

You have blockage in the drain line Take the "J" bend apart at the downstairs sink ,(see image) and run a snake up into the wall.(see image).
You will run into a bend about 8 inches in but once around that you only have to put out 6 more feet. Flush with hot water once you have installed the trap. Good luck and have a happy Thanks Giving.

John