GI「義」= That's a Grade 5 Kanji, With 2 Syllables , and 13 strokes, Including 6 combination words
Meaning :
justice; a situation which has a right and proper reason
a relative who has no blood relation
In Law
Shimai「姉妹」 = Sisters
> Conclusion GISHIMAI 義姉妹 = Yep Sister in Law, The TL was correct. The mangaka intention not using "Step" maybe there is a reason in mangaplot
Meanwhile, Step-Sister , using this kanji
Tsugi 「継」 = Inherit
Tsugi Shimai 「継姉妹」 = Step-Sisters (Too Formal)
Giri 「義理」= In-law
Giri Shimai 「義理姉妹」= Step-Sisters (informal)
#cmiiw