Missy's (Raegan Revord) Young Sheldon season 5 finale role explains why Sheldon (Jim Parsons) tolerated Penny (Kaley Cuoco) in The Big Bang Theory.
As Sheldon starts to fear the future, his concerns are eased by his twin sister, re-emphasizing how important their relationship is to his character development.
Since Young Sheldon's 100th episode, George (Lance Barber) and Mary (Zoe Perry) have been mostly preoccupied with Georgie's (Montana Jordan) pregnancy situation with Mandy (Emily Osment).
Ultimately, the constant fighting between his parents and Georgie's new role as an expecting father has caused him to develop a fear of the future.
While Missy appeared on The Big Bang Theory, she didn't really have much interaction with Sheldon to show what kind of relationship they had.
It's only in Young Sheldon that their dynamic is more fleshed out. Considering the impending tragedy of George's death, it's safe to assume that Missy and Sheldon will only be closer moving forward.
No wonder when Sheldon got separated from her sister, he fostered the same kind of bond with Penny in Pasadena.
The interaction between Missy and Sheldon in Young Sheldon season 5 finale explains why he tolerated Penny in The Big Bang Theory for many years.