The phrase "younger sister" refers to a female sibling who is born after another sibling. Basically, if you have a sister who is younger than you, she is your younger sister.Full definition
I'm going to give a shout out to two very special people, my twin younger sisters who fill my heart every day. (golubkakitchen.com)
She had a lot of responsibility from an early age, looking after three younger sisters while their parents were abroad. (sciencemag.org)
I'm — less than 5 years from 50, but look younger than my 5 years younger sister, who has only recently come to the «skincare is good» idea. (modlychic.com)