10 Signs You Know Way Too Much About That '70s Show
2. You Remember Donna's Younger Sister
There's an unintentional TV trope of disappearing kids and siblings. It's not that they die or leave, it's that they're introduced and are suddenly never used again. One of the most famous examples of this, and possibly the earliest, is on the classic series Happy Days when Richie Cunningham's brother Chuck walked upstairs in season one and never walked back down. The same thing happens on Family Matters to the Winslow's third child, Judy.
Something similar happened on That '70s Show with Donna's younger sister Tina. She appeared once in a season one episode during a party at Donna's house. After that, she is never seen again or referenced in any substantial way. The only time she even gets name checked is in a later closing credits scene that's an homage to the '70s sitcom Soap, which is more of a one off, self-referential joke.
Being that the premise of That '70s Show is conceptually similar to Happy Days, it's not hard to imagine that Tina's disappearance is a subtle homage to Chuck's.