There is no explanation given why Marlon and Shawn's mother is often referred to, but never quite makes it onto the screen. They probably just didn't find the right actress to play the mother or she wasn't written in.
The Wayans Bros. is a sitcom that aired on The WB from 1995 to 1999, starred by real life brothers Shawn and Marlon Wayans.
The series showcase the wacky escapades of brothers Shawn and Marlon Wayans, along with their wise but eccentric father.
Check out 10 Fast Facts About Marlon And Shawn Wayans at 100.7 Star.