noun
sister-in-law (spouse's younger sister or younger brother's wife)
Refers specifically to a younger sister-in-law by marriage. For an older sister-in-law, 義姉 (ぎし) is used.
義妹はまだ大学生です。
My sister-in-law is still a university student.
弟の奥さん、つまり義妹が料理を手伝ってくれた。
My younger brother's wife, that is, my sister-in-law, helped with the cooking.
noun
younger stepsister; younger adopted sister; non-blood-related younger sister
Used for a younger sister who is not related by blood, such as through remarriage or adoption. The reading いもうと is a gikun reading sometimes used for this sense, but ぎまい is standard.
父の再婚で、義妹ができた。
I got a younger stepsister through my father's remarriage.
義妹とは血のつながりはないが、本当の妹のように思っている。
My adopted younger sister and I aren't related by blood, but I think of her as a real sister.
Compound of 義 (ぎ, 'non-blood, in-law') and 妹 (まい, 'younger sister'). The reading いもうと for 義妹 is a gikun (meaning-based reading) applied to the kanji compound.