For this your sister-in-law should be educated first.
I think you can read bad effects of junk food on kids' health. Also, you can get a CD or a video where you can show how junk food affect kids' health.
http://www.livestrong.com/article/456624-negative-effects-of-junk-food-on-kids/
http://www.thenational.ae/lifestyle/family/the-immediate-effect-fast-food-has-on-children
Read more