"his (plural)" in French

“Ses” is linked to the noun that follows, not to the gender of the person we’re talking about. e.g:

Marie asks Léa: Comment s’appellent les frères de Céline ? (“What are the names of Celine’s brothers?”) Léa says: Ses frères s’appellent Julien et Oscar. (“Her brothers are called Julien and Oscar.”) because frères (brothers) is a plural word.

Marie asks Tom : Comment s’appellent les sœurs de Michel ? (“What are Michael’s sisters called?”). Tom says: Ses sœurs s’appellent Elise et Marion. (“His sisters are called Elise and Marion.”) because sœurs (sisters) is a plural word.