In English, 'my family' can be considered both plural and singular depending on the construction of the sentence. The noun can thus refer to either one unit (e.g. The family is...) or to a composition of multiple members (e.g. The family are...).
This is indeed an odd sentence, but to my ears (British-English being my mother tongue), the following sentence sounds correct: "My family (they/plural) sleep in the guest rooms (plural)". The following sentence also sounds correct: "My family (singular) sleeps in the guestroom (singular)"
The sentence that Duolingo marks as correct, however, sounds strange to me: "My family (singular) sleeps in the guest rooms (plural). Surely debateable, but I question how the family can be a singular unit if they are sleeping in multiple rooms?
British and American English differ on how they handle collective nouns like team or family. Americans use the singular and Brits usually use the plural. If you think both sound correct you're probably British.
Is there a difference between guest rooms and guest bedrooms? Is this usually how you say it in Hebrew for guest bedrooms? Or is there a different phrase? Because guest rooms sounds odd to me, I'm assuming they're bedrooms but duolingo didn't accept guest bedrooms... but what else would they be?
I'm American too (East Coast), if you were in a hotel or renting a space I can see using the term... Or if you're looking at real estate and there's a guest house or an "in-laws suite", I can see how it would be used....
However, I don't know about you, but we call these rooms in our actual home, (- our guest bedrooms -), guest bedrooms. I don't have an issue if they mean the exact same thing, except they didn't accept guest bedrooms, which is the commonly used term.