Is it permissible for a brother to question a family partnership agreement and demand a change in his share years after signing and benefiting from it, exploiting the death of the agreement's drafter?

It is not possible to rule on matters of disputes and contentions remotely by hearing from only one party. Rather, such matters must be referred to the competent authorities, such as the Sharia (definition) courts, or addressed orally with scholars. Disputed issues between relatives should be resolved through reconciliation, even if it means some relinquishing their rights for the sake of maintaining kinship ties. What the brother is doing—cursing, harming, and showing enmity—is impermissible for him. His claim cannot be ruled upon without a complete understanding of the issue and what was agreed upon when the institution was established.