She made only one or two apperacances

There are characters who disapear without alot of people realizing it.You gotta really be into a show to realize these things

It happens to a lot of shows. Sometimes the character was not liked or well received.

Seinfeld: Elaine's Dad (they said on a comentary he would have been back if he
was not so crazy)

Happy Days: Richie's older brother Chuck

Didn't Donna also have an older sister who was "in College" that was only mentioned once?

Didn't Donna also have an older sister who was "in College" that was only mentioned once?

Season 1, episode 2 (Eric's Birthday). After Red and Kitty leave the kids alone for Eric's birthday party, they go over to Bob and Midge's, play Yahtzee and Kitty starts worrying about what could happen with the kids in the house all by themselves. Midge then talks about a small book of poems that helped her cope when her daughter Val (apparently Donna's older sister) went off to college.

"The little bird that once did sing, is now alone with broken wing."

She made only one or two apperacances

I didn't even know she had a sister.