Betty knows that she's not Clarissa's main girl. But while the Harlowes give Hannah the old heave-ho, Betty knows where her paycheck is coming from. Hey, she's probably just happy she doesn't have to wait hand and foot on Miss Bratty Arabella anymore.
At least the Harlowe family trusts Betty to keep Clarissa under lock and key. When Clarissa is confined in her chamber, she's only allowed to be "in Betty Barnes's sight" (24.8). That's right—Betty always has to look over the family business.