A poster can be seen at the NBC's waiting room..
"I have to watch Sisters"
Niles mentions the show by name.
"Sisters: The Outlaw Years" is listed in the TV guide being read by Rembrandt.
Fran mentions the show by name.
Tom says that his and Crow's "cult" watches this show.
The Critic mentions this show by name.
Norman Goldman II had a bit part in this.