(a) A tax of 5% is imposed on inheritances which fall to brothers and sisters and their issue, irrespective of their being sole heirs or inheriting jointly with one of their parents, which, for purposes of this section, shall be described as Class II inheritances.
(b) For purposes of computing the tax on Class II inheritances, the taxable amount of such inheritances shall be determined by deducting from the total value thereof an exemption of $30,000, prorated to each person receiving a Class II inheritance in the ratio that each said inheritance bears to the total amount of all Class II inheritances.