When the degree of kinship is estimated with a view to ascertain the right of kinspeople to marry, the half-blood shall be counted as the whole-blood: Provided, that nothing herein contained shall be so construed as to invalidate any marriage heretofore contracted in case where by counting the half-blood as the whole-blood the persons contracting such marriage would be nearer of kin than first cousins; but in every such case the kinship shall be ascertained by counting relations of the half-blood as being only half so near kin as those of the same degree of the whole-blood (1879, c. 78; Code, s. 1811; Rev., s. 2084; C.S., s. 2496.)