It's wrong to make out with your relatives.
Mary gets drunk with her brother during family vacation, and her brother comes onto her.
Mary wants to respond to her brother.
Mary steps back from her brother and tells him to go take a cold shower.
Mary throws caution to the wind and French kisses her brother.
They wake up the next day, able to joke about the incident, because nothing bad happened.
After they sober up, Mary and her brother feel guilty and awkward around each other.