how do you get fatter legs?

Train you whole body three times per week but concentrate on your legs with squats, lunges and get on your toes to train your calves. Eat more than what you burn so you can grow muscle. Make sure you include a protein source (egg, dairy, meat, poultry, fish, nuts, lentils, beans...) in each meal/snack.

You sister is being sarcastic. Your legs are not fat at all. And honestly? They don't look good since you are way too skinny, you should put on weight and also exercise.

Eat at mcdonalds for a month