The sister
In French, the word 'La sœur' is used to represent the term 'the sister'. Just as in English, it can be utilized to refer to a sibling who is a female. The word 'la' is a definite article similar to 'the' in English, and 'sœur' is the noun for sister. One would use this term when referring to their sister in conversation or writing.
Pierre's sister is nice.
This example makes use of the possessive de (of) to show relationship. The phrase can therefore be translated to 'The sister of Pierre is nice' in English, which is colloquially expressed as 'Pierre's sister is nice'.
I am going to look for my friend's sister.
This phrase involves the verb 'chercher' (to look for) and then proceeds to use the possessive de to show relationship between 'mon ami' (my friend) and 'la sœur' (the sister). The phrase can be directly translated to 'I am going to look for the sister of my friend'.
Marie's sister is very pretty.
This example involves the use of the adjective 'très jolie' (very pretty) to describe 'la sœur de Marie' (the sister of Marie). The structure of this sentence is similar to the previous examples, using de to indicate relationship. The phrase directly translates to 'The sister of Marie is very pretty'.