The brother of one's stepfather or stepmother.(noun)
The stepbrother of one's father or mother.(noun)
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Use "stepuncle" in a sentence
"Dodi asked his stepuncle, Saudi diplomat Hassan Yassin, to go with them, but Yassin, who was staying at the Ritz, declined."
"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."
"Thus Leyel's half sister became Deet's stepmother, which made Leyel his wife's stepuncle -- and his own stepuncle -- in-law."