Suggest Remedy For Low Body Mass

I am a 21 year old female and have been extremely small my entire life. I am 5 foot 2, and 96 pounds. I have tried eating as much as I can, peanuts, sunflower seeds, bananas and peanut butter, and since I work at a steak house I eat a lot of meats (mostly prime ribs and sirloins). yet I gain not a single ounce. is there anything I can do to help? I also was thinking about drinking ensure but is it true that it causes cancer?