I just read a bit about the 1950s sitcom "I Married Joan" which was about a scatterbrained housewife by the name of "Joan Stevens" (played by actress, Joan Davis).
It was said that Joan Davis' daughter made regular appearances in the show as "Joan Steven's" sister.

I never thought that much about it, but I initially thought that Shirley Joans and her stepson David Cassidy in the same sitcom was something that had not happened before . Of course it was a step-mother and her stepson playing mother and son. I guess there had not been exactly that situation, but anyone know of any other TV show in which related people starred or co-starred, (preferably before the 70s) ?

Ronnie Burns on The Burns and Allen Show, real life adopted son of George and Gracie.