In other words, Emperor Keitai who had quiet a remote collateral lineage, displayed his legitimacy in a kind of irimuko (man who takes his wife's premarital family name) by making Princess Tashiraka as his empress, who was the younger sister of the former emperor with the right genealogy.
つまりあまりにも遠い傍系である継体天皇は、先代天皇の妹で正当な血筋を持つ手白香皇女を皇后にする事により、一種の入り婿という形で正当性を誇示したという事である。