Or do they only look at my parents’ income?
You do NOT include your brother, who you have said is your guardian. If he is a court appointed guardian, as you have said, and your parent no longer have custody of you, you’d file FAFSA as an independent and not include your parents or brother.
This is why you really need to find out the legal status with regard to your parents. It will make a big difference on whether you file as a dependent or independent. We can’t answer that question because we don’t know what kind of guardianship it is.
OP- get hold of the social worker, legal representative, or counselor who was involved in your brother’s appointment. He or she will be able to get hold of the documents which spell out exactly your legal status. Just being a guardian doesn’t tell us enough to help you.