In the season 6 finale of Young Sheldon, Missy and Sheldon have a big fight after he almost commits a crime against her. However, this kind of behavior is not surprising as it was previously explained in earlier episodes. Missy has a rebellious nature and has been disciplined before, but she doesn't seem to learn her lesson. Sheldon has also revealed secrets before and can't keep a secret or lie long-term. This theory is supported by The Big Bang Theory, where Sheldon's inability to lie is highlighted. Overall, Missy should have already known what to expect from her brother in the season 6 finale.
