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Complete the junior lawyer's notes on a bizarre inheritance case.
The judge ruled that my ___________________________ vintage car collection belongs entirely to me, but all three of my _________________________ are planning to appeal the decision tomorrow.
The correct answers are brother-in-law's and sisters-in-law.
Brother-in-law's is a singular possessive. To show possession for a compound noun, you always add 's to the very end of the word, regardless of where the head noun is.
Sisters-in-law is a simple plural. Because there are "three" of them, we need the plural form. We pluralize the head noun (sister -> sisters) without adding an apostrophe.