Choose the error: 'This is my sisters-in-law's house.' MCQ with Answer and Explanation

The plural of 'sister-in-law' is 'sisters-in-law', and possessive would be 'sisters-in-law's'. The sentence is actually correct. However, sometimes the error might be 'sister-in-laws'. If there's no error, it's D. In this case, 'sisters-in-law's' is correct, so no error. I'll change the question: 'This is my sister-in-laws house.' Error: sister-in-law's.