The '70s were a really tough time. Free love, cults, drugs. In case you don't remember, Tina Pinciotti was Donna's little sister, who appeared in only one episode. What ever happened to baby Tina? That '70s Show was a comedy, and they didn't want to walk on the dark side. Tina joined a gang and was involved in a series of bank robberies and terrorist plots. She's still in jail. In later episodes, it's insinuated that Donna is an only child. What dark family secret are the Pinciotti's hiding?
Just kidding. We don't know what happened to Donna's sister, nor did the writers even care to address it.