Is it permissible for the maternal uncle to be the guardian in a marriage contract, given that the father and paternal grandfather have passed away, and the brother has not reached the age of puberty?

The majority of scholars hold the view that a marriage contract is not valid without a guardian (wali). The order of guardians is based on their right of precedence. If your paternal uncle is the closest guardian, then the marriage contract is not valid without his permission according to the majority. The guardianship of a maternal uncle is not valid unless he is also a paternal cousin.

If the paternal uncle refuses to marry you off without a legitimate Shar'i excuse, despite the suitor being suitable, then he is considered to be "a'dhil" (unjustly withholding consent). In such a case, you have the right to refer the matter to the Shar'i judge, who will summon the uncle and order him to marry you off. If he refuses, the judge will then undertake the marriage contract himself.

A non-adult brother's guardianship in marriage is not valid.