By Emily Clement Playing at: Steppenwolf Theater, 1650 N. Halsted in Lincoln Park, through June 7. www.Steppenwolf.org Basic storyline: Carol is preparing her suburban London home for her extremely disabled son, Andy, and his 21st birthday. Her daughter, Claire, and her parents, Brian (John Mahoney of TV’s Frasier) and Pat, are coming over to celebrate. ...