A:
It is not permissible for you to use any of the minors' shares in the marriage expenses of the mature brother, even if you make him sign a document confirming their right. This is because you are required to work for
the benefit of the minors and they have no benefit in taking part of their shares to help their mature brother. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.