Besides the obvious ethical problem of snooping through mom’s private papers (assuming she keeps them in good order, etc.), this would only solve the problem for one year. The CSS would have to be filed every year the student stays in school.
OP: Your mom holds a lot of cards as to whether and where you go to school. The only way around that is to win a full tuition merit scholarship. February of senior year is a very difficult time to start hunting for those. You need to work out something with your mom, go to a FAFSA only school, or go to a school that you can afford without your mom’s information (possibly a community college or perhaps an apprenticeship or career-training program that pays you while you learn).
Please talk to your guidance counselor to see if there are any options the others here are missing.