1.
younger sister-in-law
The younger sister of one's spouse, or the wife of one's younger brother.
義妹は大学生だ。
My sister-in-law is a college student.
義妹と一緒に買い物に行った。
I went shopping together with my sister-in-law.
夫の義妹が来月結婚することになった。
My husband's younger sister is getting married next month.
2.
younger stepsister
A younger female sibling by remarriage of a parent, not related by blood.
父の再婚で義妹ができた。
I got a stepsister when my father remarried.
義妹とは血のつながりはないが、本当の姉妹のように仲がいい。
Although we're not related by blood, my stepsister and I are as close as real sisters.
義妹 uses the character 義 meaning "in-law" or "adopted," indicating a non-blood relationship.
RELATED FAMILY TERMS:
- 義兄 — older brother-in-law / older stepbrother
- 義姉 — older sister-in-law / older stepsister
- 義弟 — younger brother-in-law / younger stepbrother
In conversation, speakers often say 旦那の妹 (husband's younger sister) or 嫁の妹 (wife's younger sister) to be specific about the relationship, since 義妹 can mean either in-law or step-sibling.