A contract to enforce weight loss

We were sitting at a pizza restaurant last night, not quite recovered from our trip, and heard a woman behind us saying that she had written a contract with her brother saying that if she hadn't lost 40 pounds by next year, she'd have to pay a $10,000 penalty.  Her brother had also promised, in the contract, to sue her.  

Oh, man,  what a contract law exam question, with a little embellishment.