It is just reasonably doubtful that Jacob had exactly that twelve sons we were told. The whole Abraham, Isaac, Yacov story is a made up ethnogenesis, an artificially created hindsight. The Bible story shows that in order to create twelve ancestors of the twelve tribes they had to get kids not only from Yacov’s wives, but from his maidservants, meanwhile the maidservant of Abraham and her child Ishmael, were ousted from the lineage and from the tribal structure together with the six children of Abraham’s Canaanite wife, Keturah.