What is to be done with a religious brother who is shunning his family, and what is the ruling of the Sharia on that?

It is obligatory for the brother not to sever ties with his kin, for breaking family ties is a major sin, and the texts of the Qur'an and Sunnah have condemned those who do so. It is obligatory for one whose kin have severed ties with him to initiate reconnection, because this is the true connection, as the Prophet (peace and blessings be upon him) said: "The one who maintains ties is not the one who reciprocates, but the one who maintains ties is he who, when his kin sever ties with him, reconnects with them." It is obligatory for the questioner to respond to his brother's ill-treatment with kindness and to his severance with connection, as Allah Almighty says: (Nor can a good deed and a bad deed be equal. Repel [evil] with that which is best, then verily he, between whom and you was enmity, [will become] as though he was a close friend).