"I couldn't marry him," said Debra. "I wouldn't be able to tolerate my two _____ attitudes." sisters-in-law's sister's-in-law sisters-in-laws' sisters'-in-law
Sister-in-laws' because it is plural not singular. When a noun ends in an 's' whether it is plural or that is just the way it is spelled, you add the apostrophe at the end of the word and no "s".
