https://www.cbr.com/best-drop-in-sitcom-characters/

Family Matters' Steve Urkel and Seinfeld's Kramer are some of the most popular unannounced visitors in sitcoms and there are a few others like them.

Marie and Frank Barone. Especially Marie.

Uncle Arthur
Serena
Dr. Bombay
Aunt Clara
Maurice
Phyllis Stephens
Jeannie's Sister
Djinn-Djinn
Cousin Itt
Ophelia Krump
Cousin Pearl
Jethrene
The Great Gazoo
Hymie the robot
Uncle Tounoose
Homer Bedloe
Elverna Bradshaw
Selma Plout

Janice from Friends seems obvious but missing from the list.

Do Kramer or Raymond’s parents even count given that they were part of the main cast of their shows from the beginning.