I am deeply moved by your desire to help the family. Normally, I would encourage someone 16 to worry more about finding ways to take care of themselves — and plan to move out or on to college. But there are several differences in your situation that I believe warrant something other than a more standard reaction. Your father’s abuse triggered a family response. Your grandparents took you all in and your mom did what she could do for money and put the focus on her children. Each of these responses is, considering the situation, wonderful.
The tipping point in your reaction is the level of appreciation and understanding. You value your grandparents, your mother, your mother’s boyfriend, and have a sense of deep caring for your siblings. At the same time you recognize that you have a plan of your own, and that each of the people around you has limitations.
In this instance I would look to find assistance in the form of counseling, vocational guidance, and housing for your mom and your siblings. The best place to begin is with agencies that are known in your area like this http://www.downtownwomenscenter.org/health. They are woven into the community and while they might not be the right ones to deliver exactly what you need — they can certainly point your family in the right direction for getting help.