This Isn't The End (1970)

The story follows a quiet, introverted girl Quinn who lost her mom and hasn't been the same since then. She retreats to her room, while her step-brother Mitch is loud, popular, captain of the football team, and always smiling. When Mitch decides to throw a party while their parents are away, Quinn bumps into Donny, her step brother's best friend, with whom she quickly forms a connection with. However, things escalate during the party between Mitch and Quinn and we quickly see that there is much more behind Mitch's perfect smile.