No. A legal guardian is someone who has a court order giving them custody of a child, or the authority to make decisions about the child.
There is usually no need for a guardianship unless the parents are unwilling or unable to care for the child.
A legal guardian can have almost all of the authority a parent would, depending on the order.
Your big brother is your big brother, not your legal guardian.
Please understand that this is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like Avvo, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.
