Words like “my”, “your”, “his”, “her”, “its”, “our”, and “their” are possessive adjectives. They modify nouns to indicate ownership and association. Like in English, they precede the nouns they modify:

   mon frère
   my brother

   ton frère
   your brother (familiar)

   son frère
   her brother

   notre frère
   our brother

   votre frère
   your brother (formal)

   leur frère
   their brother

However unlike English, they also change gender based the object it modifies, if in the first- and second-person forms.* The feminine forms are listed below:

   ma soeur
   my sister

   ta soeur
   your sister (informal)

   sa sœur
   her sister

However, there is an exception for pronunciation sake. If the feminine noun begins with a vowel, the masculine possessive adjective is used:

   mon amie
   my (girl)friend

   son amie
   his (girl)friend

With plural nouns, gender inflection is ignored and neutral plural forms are used:

   mes soeurs
   my sisters

   mes frères
   my brothers

   tes sœurs
   your sisters (informal)

   ses sœurs
   his sisters

   ses frères
   her brothers

   nos sœurs
   our sisters

   vos frères
   your brothers (informal)

   leurs sœurs
   their sisters

It is important to remember too, that French adjectives inflect for gender based on the noun being modified. In English possessive adjectives (his/her/its) also inflect based on gender, but do so based on the gender of the possessor.