In Young Sheldon season 6, Sheldon and Missy are finally getting along onscreen. This was something that never happened in The Big Bang Theory, where the pair rarely saw each other and were always at odds when they did. It's easy to see why The Big Bang Theory never capitalized on Sheldon's friendship with Missy since Penny was too similar to Missy. This meant that Sheldon's interactions with his sister would have been redundant. However, like how the spinoff made up for The Big Bang Theory never mentioning Georgie's children, Young Sheldon season 6 is currently fixing this unfortunate oversight from its predecessor.
Young Sheldon season 6 has repeatedly proven that Missy and Sheldon can have fun as unlikely friends, while The Big Bang Theory failed to allude to this. In Young Sheldon season 6, episode 17, "A German Folk Song and an Actual Adult," Sheldon offered to give Missy Beverly Hills 90210 recaps since she was grounded and not allowed to watch television. This led to the unlikely friends sharing an interest in the show, a surprising development that was foreshadowed earlier in Young Sheldon season 6. Before realizing Missy had gone on the joyride that led to her grounding, Sheldon reached out to confide in her about his database's failure via walkie-talkie.
From stories like Sheldon finding a workaround for Missy's TV ban (and learning about social cues from Beverly Hills 90210) to more poignant moments like his attempt to gain sympathy from his sister, Young Sheldon season 6's Missy/Sheldon moments humanize the pair. Both characters feel more fleshed-out when they share storylines, as proven when Sheldon's frustrated attempts to locate his sister allowed Young Sheldon to explain one Big Bang Theory character flaw. Missy also shows her more patient side when interacting with the socially challenged Sheldon, giving the title character more grace than he frequently receives from colleagues or the rest of the Coopers.
In The Big Bang Theory, it seems like Sheldon doesn't like his sister or care about her life, but this is not necessarily the case. Life is composed of seasons and, while the twins might have been close confidantes in their childhood and adolescence, it makes sense that Sheldon's move away from Texas and Missy's decision to remain in their home state made the duo a little more distant. However, Young Sheldon season 6 can clarify through these sweet subplots that the siblings are fond of each other and simply accepted later in life that they chose very different paths.
The biggest problem with Young Sheldon's Missy story is that the character was never intended to be taken very seriously in The Big Bang Theory, so the fact that she ended up in what Sheldon viewed as a suboptimal living situation wasn't particularly sad. However, Young Sheldon has made Missy much more human, making her fate far sadder. Fortunately, Young Sheldon's future stories can prove that, despite choosing a different life from her, Sheldon does still love his sister and, despite her life not aligning with his ideals, Missy is still happy with her fate by the time The Big Bang Theory takes place.