If any member of such fire department being single and unmarried shall, while in the performance of his duty, be killed, or die, as the result of an injury received, or shall die of any disease contracted by reason of his occupation as fireman, or shall die from any cause whatever while in said service, and shall leave a father or mother who are dependent upon him for support, or a brother or sister under the age of sixteen years so dependent, said board of trustees shall direct the payment from the pension fund monthly to such dependent parents or the survivor thereof and to each such dependent brother or sister under sixteen years of age, such sum of money as may be determined by the rules and regulations of said board; provided, that the board of trustees shall also have the power to make provision for any such dependent brother or sister over the age of sixteen years who may be mentally deficient or physically incapacitated, in such sum as the board my determine, and the board shall have the sole power to determine who is mentally deficient or physically incapacitated.