It is bad to betray those you love
Alana is good friends with Josh, her sister's boyfriend, who has invited her to a party.
Alana wants to attend the party with Josh.
Alana asks her sister if it'd be alright to attend the party with Josh.
Alana and Josh go to the party without telling her sister.
Alana's sister is fine with the idea and wishes them a good night.
Alana's sister loses trust in her, straining their relationship for a while.