I agree with mommieof2. Try to have a calm talk with your mom and maybe ask if she could put certain items on her shopping list that fit in with your plan. Play it low-key and don't act the martyr, just do your thing - nobody can force feed you foods you don't want to eat!!
In the meantime, you could just eat the healthier things at the bbq. Don't make a big deal about it in order to make your mom feel bad - she'll just get her feelings hurt and won't want to co-operate with you. You can have some chicken - just take off the skin and have just one or two chicken legs. If you have a hamburger, try it without a bun or with half a bun and plenty of fixings like lettuce, tomato and onion. If your mom didn't buy these kind of toppings maybe you could volunteer to go the store and get them - they're not very expensive. Eggs are a healthy food, you can make an omelet or scrambled eggs with two egg whites and one yolk (87 calories) - good for you and lots of protein which is very filling. You can even have one slice of well-cooked bacon (so that most of the fat is cooked off) with it for 43 calories.
If your mom won't buy lowfat or nonfat milk, try using less milk on your cereal. Be sure to measure the cereal serving, though, as most cereals are very high in calories (you probably know these things, as you say you've lost weight and are trying to maintain).
Instead of soda, you could just get yourself a nice tall glass of iced water or make yourself some iced tea - it's easy to do!
Congratulations for trying to stay on track even when you don't see eye-to-eye with you mom about food.