WordNet
foster-sister
n. your foster sister is a female who is not a daughter of your parents but who is raised by your parents
Usage examples of "foster-sister".
McCool goes and gives the money to his foster-sister, Miss Arrah Meelish, who is goin to shortly marry Shaun, the Lamp Post.
He had managed to annoy everyone, provoking his sister Yllana, his foster-sister Alanna, and Ida Davidson, who was usually impervious to the behavior of adolescents.
It has even been used on me: My foster-sister Arianeira used it to break the Curtain Wall and let the Imperial armadas into the Bawn of Keltia.