This is thought to be because of a relationship that the father of Umayado was her elder brother, Emperor Yomei, and also his mother was her younger paternal half-sister (and their biological mothers were sisters), and thus Umayado was her most trustworthy relative.
厩戸の父は推古天皇の同母兄の用明天皇、母も異母妹（かつ生母同士が実の姉妹関係）の間柄であり、竹田皇子亡き後において厩戸が天皇にとって最も信頼のおける血縁者であったからだと見られている。