Her Sister in French
Her sister In French, sister is called “sa soeur”. Sa is a possessive pronoun which refers to something or someone you own; typically this phrase is used with nouns such as daughter or sister but can also be used on its own as in “sa femme.”
French has an expansive and complex vocabulary for in-laws and step family members, which can sometimes be confusing as some words switch between masculine and feminine meaning – such as mother and sister in laws (mere et soeur) as well as father & brother / frere.
Alternatively, use context to help determine its meaning. For instance, hearing about a belle-mere in a movie or TV show probably refers to her mother-in-law whereas hearing a story about her younger sister may mean step-sister.