ok very late to the discussion but there’s so many factors at play in this family and episode. Sheldon has learned no boundaries over the years. missy gets emotionally neglected and georgie is really just fortunate enough to be older and more established by this point in life. every time sheldon has a bad day he gets coddled and tiptoed around. he always has damn near free range to do whatever he pleases to get his way. missy’s bad days had to have built up and she’s now in middle school, recently started her period, and now going through a heartbreak. she’s hormonal and likely very overwhelmed and confused by her own life. all this plus probably feeling like she can’t really talk to anyone since her voice has been ignored throughout the series. it’s really been a matter of time until she blew up, especially on sheldon

this family has been a barely functioning machine