Hacking your brother’s email account and sharing all his secrets with your mom could be considered an act of personal treason, meaning you have betrayed your brother.
The word treason can be traced back to the Latin word traditio(n-), which comes from the verb tradere meaning “to hand over, betray.” If you commit treason, that’s what you do: you betray. The word is typically used to refer to the betrayal of a government or a country, like when a spy shares national secrets with another country, but it can be used to refer to personal betrayals as well.