Gwen -
People who are trying to lose weight do need to think about nutrition, but so do people of healthy weights! Nutrition isn't solely related to weight loss - just having a small number on the scale doesn't mean you're getting adequate nutrition for your body. So, in my opinion, there is no need for you to "buy things for them" that are junky. One suggestion might be to sit everyone down and say "I'd like to work on eating more healthy foods in the home. If you want a snack food that isn't here at the house, you can buy a single serving of it, but I'd prefer it if you didn't bring home a whole package. I want us all to eat more healthfully together!" You're not depriving anyone - they can still eat junk if they so choose - but you're limiting your own access by telling them to buy single servings. In addition, you're putting the focus on nutrition, which will be valuable for your whole family, rather than on weight loss.
Anyway, I'm glad you've joined us. Settle in and keep posting - can't wait to hear more from you soon!