mon aînée

my older sister

→ mon
phrase Rare #44,863
my older sister Refers to an older female sibling. The masculine possessive adjective mon is used before aînée because it begins with a vowel sound, even though aînée is feminine.
FR: Ma sœur est mon aînée de trois ans.
EN: My sister is three years older than me.

Je pensais que tu étais mon aînée.

I thought you were my elder.

Il est marié à mon aînée, il vient de rentrer.

He is married to my eldest daughter, he just got back.

— Silent Voice (2016)

Je pensais que vous étiez mon aînée.

I thought you were my elder.

Elle est de six ans mon aînée.

She is six years older than me.