Does the younger brother have the right to claim his share of the proceeds from the sale of a leased plot of land after thirty years, knowing that his siblings had relinquished it to the eldest brother, who was the sole farmer?

The leased land is not an inheritance for your mother, and the law of perpetual lease is void and contrary to Sharia, which stipulates that the lease term must be known and specified.

Therefore, the "khalw" (premium for vacating) that you received from the other farmer in exchange for relinquishing the land is not permissible for you. This money must be returned to its owner if he is alive, or else to his legal heirs. If that is not possible, then it should be given as charity to the poor and needy, with the intention that the reward of the charity goes to its owner. If its owner appears after that, he is given the choice between approving it, in which case he receives the reward, or taking his right, with the reward going to the one who gave charity.