姉妹【しまい】
sisters
妹さん【いもうとさん】
younger sister
兄妹【けいまい】
older brother and younger sister
弟妹【ていまい】
younger brother and sister
義妹【ぎまい】
sister-in-law (spouse's younger sister or younger brother's wife), younger stepsister, younger adopted sister, non-blood-related younger sister
実妹【じつまい】
biological younger sister