Ray Romano's real-life brother has a cameo as Robert's partner. He's accompanying Robert as he approaches Amy to propose to her.
Amy's brother Peter is wearing an "East Island Cheetahs" T-shirt. This shirt is from a high school on the eastern Caribbean island of Saint Lucia.
In season 4 episode 17, Amy's brother is named Russell and is played by Paul Reubens. In this episode, he is called Peter and is played by Chris Elliot.