I agree with Sasha N that your daughter will eventually encounter a situation that makes her feel insecure and it's up to YOU as her mother and female role model to equip her mentally/emotionally to deal with this. I think that if you lead a healthy lifestyle by eating well (not obsessively well, but lots of fruits, veggies, healthy carbs and the occasional treat)and get exercise (by walking, playing outside, swimming, etc.), your daughter will learn by example. As the other moms stated in their responses, it may also be a good idea to speak to your in-laws about the situation. True, it may be uncomfortable, but hopefully you may be preventing future problems. You could try to say, "I know you're concerned about watching your weight, but I would prefer if you didn't talk about that kind of stuff around my daughter." You can also add that some (good) fat is essential to a child's health (brain food!). You may also consider getting your daughter involved in some kind of sport (gymnastics, soccer, swimming, etc.). Studies have shown that adolescent girls who are involved in sports often have better self-images.
Good luck! You are doing the right thing!