"They are my brothers' wives." means there are some women, and they are the wives of my brothers. Each brother has one wife. No brother has more than one wife. No hint of polygamy is intended here The clue is in " brothers' " with s + apostrophe referring to more than one brother.
I guess in the singular it would be:
Jsou to ženy mého bratra
Correct or incorrect?
Not necessarily, depends on the context. Does the Czech sentence imply that all the respective wives are present? Or could the sentence also refer to some but not all wives? If the latter, then “those/they are wives of my brothers” could be a possible translation.
No, not all. Those women who are standing over there (for example) happen to be (the?) wives of my brothers.
I wrote this: They are my brothers' wives. The correction showed that I wrote this: They are my brothers wives.
It does. Duolingo has sometimes problems with apostrophes in word bank exercises.