I have to agree with your parents. That really doesn't seem that bad. And you will be growing and changing more for the next few years.
But you are really at a great time in your life to start making food and exercise habits that last a lifetime. Start doing research and reading about what healthy eating means. You can't do anything about the bad food in the house, but you could try learning to cook healthier foods.
As far as exercise, could you walk, run or bike to or from school? Even part of the way? Does your school have a gym or weight room you could use? What about exercise classes given before or after school? Many of these classes are mostly female so it would be a great way to meet girls. It's fall and cross country season, could you join? Does your school have a pool? Swimming is a great way to develop nice chest and shoulders.
If you start learning to exercise and eat right now in 15-20 years your brothers may be coming to and asking how they can get rid of the weight they have from eating too much junk food. Try to approach this as an opportunity instead of as a chore.