When someone is unfaithful, they're disloyal or unreliable. If you don't honor the pledge you made "to help people at all times," you're being unfaithful to the Girl Scout code.
While it's common to use the word unfaithful for people who break their marriage vows, it's a great way to describe someone who's broken any serious promise or neglected an obligation. An elected government official who doesn't keep their promises to voters is an unfaithful public servant. And if your brother refuses to participate in the family's annual Halloween custom of dressing as pirates, you can say he's being unfaithful to the tradition.