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 can I say - she did a great job! We had a couple of ladies in our group who thought that it was much easier to pay in kind to the professor than to be up all night cramming incomprehensible formulas and dates. But here, as they say, it is a matter of what you learn!