/ Dictionary / Index F foster sister: Meaning and Definition of Find definitions for: fos'ter sis'ter Pronunciation: [key] a girl brought up with another child of different parents. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. foster parent foster son Related Content Daily Word Quiz: mamba Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language