The sister of one's stepfather or stepmother.(noun)
The stepsister of one's father or mother.(noun)
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Use "stepaunt" in a sentence
"That is, it was provided at no cost to the Maxons by the parents of the groom, Regina and Milo Minderbinder, to express their love for their new daughter-in-law, their undying friendship for her stepuncle and stepaunt, Christopher and Olivia Maxon, and their deep gratitude to every single person present who had taken the trouble to come."
""I'm sure a girl who looks as much like you as I do must be at least as much relation as a stepaunt.""
"There were people who believed that he had run away again, this time never to return; among those who held to this opinion being Mrs. Louden and her sister, Joe's stepaunt."