Completely unrelated linguistics tangent that nobody asked for (but I spent more than 3 semesters specifically learning): Quite a few of the romance languages (that is, those that evolved from Latin), have gendered pronouns and articles (el/ella/ello-la/el/lo-un/una in Spanish, il/elle-le/la-un/une in French); however, when a noun starts with a vowel, the feminine pronoun/article discards its final vowel to accomodate.
This led to a very funny thing with the word "water": in Spanish it evolved from the Latin word aqua, and its infinitive form was "Illa aqua", over time the double A became bastardized and the result was "Il aqua". So in Spanish we use "El agua", in masculine, when talking about the singular, but "las aguas", in feminine, when using plural. It doesn't actually mean the word changes genders, as you'd still say "el agua clara" (the clear water), keeping the noun feminine and using the feminine form of the adjective, but the masculine form of the article.
In THIS case, it's because French has two forms of the singular "his/her/their" pronoun. Which is kinda funny because the gender being pointed out in "sa/son" is the person/object being DESCRIBED, while in English it refers to the person/object it BELONGS to (well, belongs is the wrong word, but I'm struggling to translate it right now). "sa soeur" (his/her/their sister) is thus grammatically correct, but "son soeur" sounds weird. Apropos of nothing, in Spanish we only use the possesive "su" for singular, and "sus" for plural; no gender either way.