What is the ruling of the Sharia regarding the right of orphaned, underage siblings, whose share in their father's shop was relinquished, after they have grown up and demanded their right from their eldest brother, who claims he was spending on them and the family?

To resolve disputes, both parties must be heard. The matter should be referred to a court of law or to an arbitration by a knowledgeable and religious person whom you agree upon, and whose judgment will be binding.

If the brother is spending from his own money, then waiving your share is a void sale due to the unknown price. Thus, the shares remain the property of their owners. They must reimburse the brother for what he spent if he was not donating it, and he is obligated to give them their share of the shop's income.

Since the brother admitted that the expenses for study and marriage were covered by the shop, which you denied, this confirms the necessity of hearing both parties and examining their claims and evidence, while observing kinship ties.