By saying that he doesn't have a daughter, can she mean that he has a son? Why doesn't she say simply that he doesn't have children? Is she just playing a bit with her fans?
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I've never thought of Snape as having a family - as a wife and kids - only possible siblings or parents, but never children. Would he have a son? Who would have married him? Okay, you don't have to be married to have a son, but... you have to have someone to have him with... and although I like Snape a lot, I can't imagine him being in any relationship.
A few of the Hogwarts professors have spouses, but the information is restricted – that much we already know. If Snape had a son, could Voldemort use him against him (if he were to find out that he's no more loyal to him)? But JKR said once that she thinks that the thought of Snape in love is a "very horrible idea" and is stunned that someone wonders if Snape might fall in love. Therefore I've always thought that he's never been in any relationship... then again, you don't need to fall in love to get married...