Did Rachel McAdams' sister play her sister in The Vow?

No. Gwen, Paige's (McAdams' character) sister is played by Jessica McNamee, who is the younger sister of actress Penny McNamee - they are from Australia :) Rachel McAdams is also VERY pretty. She practically pulls off any hair color. Check out some of her movies----my favorites are "The Notebook" and "Mean Girls".