Well, this should be easy.
Prokop will play Ethan, the nephew of one of Claire's friends. Described as a stand-up kid, Ethan is moving to town to go to college and Claire (Julie Bowen) thought it couldn't hurt to introduce him to Hayley (Hyland).
This won't be the first time the real-life couple has appeared on screen together. They played a couple in the sex comedy Conception and starred together in Disney's Geek Charming. They'll both also appear in the movie Struck by Lightning, written by and starring Glee's Chris Colfer.
Prokop will appear in one episode slated to air later this year. Modern Family airs Wednesdays at 9/8c on ABC.