The Selwyn Case

'For a long time, it was assumed that Salazar Slytherin was an only child but recent studies conducted by Mops et al. (1941) suggested that he had at least one sibling, potentially a sister, whose first name remains unknown but who is believed having married under the name of Selwyn'

Campbell, M. (1942). A Tormented Wizard: Life and Death of Salazar Slytherin. Perpignan: Beauxbatons Academy Press.
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Or how Tom Riddle's thirst for power brings him to marry that girl.
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This story contains references to sexism and racism towards non-magical beings. The author strongly condemns any form of racism, sexism, homophobia, transphobia, ablism and other forms of discrimination against unprivileged groups.