I'm mad at my parents right now.

My sister is 13 years old. She's a bit big. Like 5'2" and 140 lbs. Living around my parents is a particularly stressful occupation and a few years ago she turned to food as a sort of comfort mechanism.

Since then, my parents have been nagging her to lose weight. They would do all kinds of stuff to her like yell at her for eating so much, or say she could only watch tv if she was on the treadmill, etc. Finally they took her to a personal trainer like 6 months ago, and she's been doing the low carb diet with my mom, and she's lost some weight.

My sister has been nagging my parents to get a dog for a long time now. They have a particularly wussy cat which she smothers with affection but apparently one pet isn't enough for her. They've continually said no for a few reasons: a dog will make the cat's life miserable, she can't take the dog with her to college, etc.

Today my sister told me that she might be getting a dog. What brought about the sudden change? She made a deal with my parents that if she gets down to 120 lbs then they'll get her a puppy so it will grow up with the cat and not terrorize it. Signed a "contract" and everything. Motivation to lose weight is one thing, but I'm afraid that she's so dead-set on having a freaking puppy that she's going to go overboard trying to lose weight.

Please, someone else tell me that my parents are wrong for making this deal with a 13 year old girl.