"They are my sisters' husbands."
Translation:To jsou manželé mých sester.
Regarding the word manželé in other contexts, is it a noun that refers to a married couple? Does it only refer to a husband and wife couple, or can it refer to same sex couples? Would the root word be different if it were in reference to two women married to each other? Thanks, mods!
The terminology is not well established for same-sex marriages. There is no same-sex marriage in the Czech Republic. There is only registered partnership (but it has been around for a long time). I would find manželé very strange for two women, unless the word was used for many couples including also some women-women ones.
"To jsou manželé mé sestry." would mean that they are all her husbands, she being just one sister. That does not work with the placement of the apostrophe in the English sentence, which has my sisters' husbands rather than my sister's husbands.
Perhaps you mistook "mé sestry", the singular genitive form, for some of the plural forms that look the same?