Is the sister entitled to demand the return of the money she gave her brother as assistance for marriage, knowing that she intended it to be from zakat funds, but scholars denied the validity of that, and that the brother has spent the money on his marriage but possesses an amount to cover the repayment despite not being wealthy? And is it permissible for the brother to charge a fee for services he rendered to his sister with the intention of returning the favor after his sister denied this favor?

The sister is not permitted to demand her brother return the money, whether it was a gift or zakat (definition). This is because it is not permissible to revoke a gift or charity after it has been taken possession of, except for a father regarding a gift, and with the exception of a gift of reward (hibat al-thawab) where the giver did not achieve his objective. The Prophet, peace and blessings be upon him, likened the one who revokes his gift or charity to a dog that returns to its vomit. It is permissible for the brother to consider the services he provided to his sister as a way of reciprocating kindness, if the money was a gift given with the intention of reward and his sister did not reward him for it.