Definition of 妹背 (いもせ)

noun
1.
couple, husband and wife, man and wife(archaism)
2.
sibling, brother and sister(archaism)
Other readings:
妹兄【いもせ】
Related Kanji
妹 younger sister
背 stature, height, back, behind, disobey, defy, go back on, rebel
兄 elder brother, big brother