Specifically refers to one's spouse's older sister or one's older brother's wife. Polite term used within the family or when speaking about her to others.
I heard that my sister-in-law is getting married next month.
My sister-in-law had a baby.
older stepsister; older adopted sister; non-blood-related older sister
Used for an older female sibling not related by blood, such as a stepsister, adopted sister, or a close family friend treated as an older sister. Less common than the sister-in-law sense.
She is my older stepsister, the daughter of my father's new wife.
I've called her my older sister since I was little, though we're not related by blood.