"your (formal singular)" in French

“Votre” agrees in number with the noun that follows, not with the number of people we’re talking to. It is the same in both masculine and feminine forms.

e.g.:

Marie asks Léa and Tom: “What’s the name of your brother?”(Comment s’appelle votre frère ?) because Léa and Tom only have one other brother (frère).

Marie asks Tom and Jerry: “What’s the name of your sister?”(Comment s’appelle votre sœur ?) because Tom and Jerry only have one sister (sœur).