Her sister got hysterical over her boyfriend who drew a slutty picture of her - how skinny and flat she was. Her brother calmed her down and measured her waist and hip size, assuring her that she was awesome! Sure, her gratitude was inadequate - sucking her brother's cock, but didn't the girl deserve sympathy? When she wanted to remove her head already, he wouldn't let her - if she wanted to be a grown-up, then swallow it. And it seemed his sperm was to her liking. Now she could always count on him.
What a seductive ass her sister has, you can tell right away that her sister is into it. No wonder the bat fucked her. Sister's titties are pretty tight, too.