Are the parents and siblings sinful for shunning their disobedient son and brother, who mistreats everyone and prays only a little?

The boycotting of a disobedient person should consider what is in their best interest. If boycotting will lead them to repentance, then it is prescribed. Otherwise, it is better to be patient with them, continue advising them, and seek advice for them from someone who influences them. As for an abusive relative, one should be patient with them and repel their harm with good deeds, in accordance with the Almighty's saying: "Repel [evil] with that which is better." If harm is feared from them, there is no harm in avoiding meeting them without a complete severance of ties. It is permissible to threaten them with a complaint to the authorities if that would affect them, or to seek ruqyah for them and psychological treatment.