After just finishing up his summer tour over the weekend, singer Brian McKnight is preparing to make his small screen debut with a guest spot on the WB sitcom "Sister Sister."In the show, McKnight will play a graduate student who is sought out by one the show's stars. McKnight's episode is currently scheduled to air in September during the first month of the new TV season. McKnight's first post-tour duty will be to co-host the Soul Train Lady of Soul Awards with Brandy and L.L. Cool J on Thursday night in Los Angeles.