Donna actually has two sisters: Valerie, mentioned in "Eric's Birthday" (season 1, episode 2) who went off to college the year before. She is never mentioned again; and Tina, younger, who is seen in "Eric's Burger Job" (season 1, episode 5) then never seen again. She is mentioned in "Vanstock" (season 2, episode 6) in the voiceover in the end credits, where the narrator asks something along the lines of "What ever happened to Donna's younger sister, Tina?" After that, she' never mentioned again.
And then the writers turned Donna into an only child, ignoring the fact that an older sister had been mentioned and a younger sister seen. In "Time Is On My Side (season 7, episode 1), Kitty says: "But, Red, if Bob picks Midge, he will re-unite a shattered family, bringing their only daughter a glimpse of happiness for the first time in her life."
So, no, you didn't miss anything. They created a couple of sisters, then pretended they didn't. Just like the timeline, it's messed up.
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