Betrayals play a critical role in the play and show the workings of wickedness in both the familial and political realms—here, brothers betray brothers and children betray fathers. Goneril and Regan’s betrayal of Lear raises them to power in Britain, where Edmund, who has betrayed both Edgar and Gloucester, joins them. However, the play suggests that betrayers inevitably turn on one another, showing how Goneril and Regan fall out when they both become attracted to Edmund, and how their jealousies of one another ultimately lead to mutual destruction. Additionally, it is important to remember that the entire play is set in motion by Lear’s blind, foolish betrayal of Cordelia’s love for him, which reinforces that at the heart of every betrayal lies a skewed set of values
"Our flesh and blood, my lord, is grown so vile, that it doth hate what gets it." (Gloucester to Lear about both their children.)
Lear feels as though Cordelia betrayed him.
"I loved her most and thought to set my rest upon her kind nursery. Hence and avoid my sight!" (Lear about Cordelia)
Brothers betray brothers (also sisters betray sisters)