Concept:Compound nouns like "sister-in-law" usually form their plural by making the main noun plural.
Explanation:The word "her" is a possessive determiner, so the blank must be filled with a plural noun, not a possessive one.
The correct plural of "sister-in-law" is "sisters-in-law" because we pluralise only the main word "sister".
Option C is wrong because it adds "-s" to both parts: "sisters-in-laws".
Option B, "sister's-in-law", is a singular possessive form.
Option D, "sisters'-in-law", wrongly places the apostrophe after "sisters".
Thus, the right choice uses the plural form without any apostrophe.
Answer:A. sisters-in-law