www.acronymfinder.com does include Yr. as an acronym for Younger. According to the Oxford English Dictionary, "Younger" is "Used after a person's name for distinction from an older person of the same name" particularly in the titles of Scottish heirs. However no mention of gender is made. By the way both 'Younger' and 'Junior' are derived from the Latin 'Juvenis', meaning "young".
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