Hi 0171292,
I think in this case it sounds like you're going to have to rely on yourself

What is motivating you to go to the 6th form? Do you have any career plans? Maybe if your parents can see you've got an aim, they might leave you alone for a bit. Are you in lower or upper 6th? If you do as well as you want in your AS levels, you'll have something to show them then. And your results are tangible too, because you can apply to uni with them. Your parents might not complain so much when you can say you've got offers from several unis, lol. Can you get your brother to support your plan to go to uni?
What subject do you want to study at uni?