With basic French adjectives, if the subject is singular and masculine, like "mon frère" you would add nothing to the adjective (e.g. mon petit frère). If the subject is singular and feminine, like "ma sœur", you would add an "e" to the adjective (e.g. ma petite sœur). If the subject is plural and masculine, or a group of mixed genders, you would add a "s" (e.g. mes petits frères). If the subject is plural and feminine, you would add an "es" (e.g. mes petites sœurs). Also, if the masculine adjective like "aimable" already has an "e" ending, you would add nothing for singular masculine and feminine subjects (e.g. mon frère est aimable; ma sœur est aimable). You would add an "s" to the end of masculine and plural subjects though (e.g. mes frères sont aimables; mes sœurs sont aimables). ok sorry for being really lengthy I hope that helps.
