What is the ruling of Islamic law regarding a deceased mother's will that stipulated that one of her sons should not attend her washing or funeral, due to her anger with him, and is there any sin upon the son who carries out this will?

Part of filial piety is fulfilling the parents' covenant after their death, based on the hadith of Abu Usayd: "Praying for them and seeking forgiveness for them, fulfilling their covenant, honoring their friends, and maintaining the ties of kinship that are not kin to you except through them." You must admonish your brother and command him to repent, because disobeying parents is one of the greatest sins.