Joseph was of no relation to Yeshua, thus had he any children of other women (which he did not, he was a Godly and upright man) they would be of no relation to Yeshua.
Under Jewish law and tradition an adopted son was a closer relation than a biological relation. An adoption was forever, but a biological child could be disinherited. It is a foretelling of the Gentiles becoming the adopted children of God. It is only modern culture that puts such an emphasis on biological paternity.