When you are talking about family members it's ok to omit the definite article (both in Hebrew and in English) - "Mom is in the kitchen" = "אמא נמצא במטבח/אמא במטבח". The assumption in these sentences is that you are talking about your family members.
It's possible to translate the sentences as impersonal or generic - "some uncle is cooking meat", "an uncle is cooking meat", but I'd say that this meaning is less obvious than the former.
I am a native Hebrew speaker and a linguıist.
The analogy with Uncle Aunt is inexact. Mother Father have a special syntactic(!) role in almost all languages (did you know that they are the nouns that most commonly do not need a conjunction ("and") to join? as in "the mom dad duo" "אמא אבא ... ", some languages even conjoın them to a special word such as annebabamle etç...) to a point that it somewhat affects the knowledge assumed over them, the same cannot be said for Uncle Aunt.
I have to assume that this is a sentence spoken at me and as such it has no possible definite article reading.
That said, this reading can be introduced in a slang equireading of "el tio y la tia estan cosinando carne" "un tio y una tia estan cosinando carne" but in a very informal hip hop like setting in some idiomatic sense, I don't think this is what Duolingo wants me to imagine when using the app.
and in any case the more salient meaning should obviously always be allowed.
Ok, I see what went wrong now,
I agree that "uncle and aunt ..." is a possible interpretation though this sentence is not very plausible in English, after entering "an uncle and an aunt..." I was corrected by Duolingo automatically with the (most similar) sentence "the uncle and the aunt ..." I think this definite article reading is almost impossible, mine is very implausible in Hebrew and the one without any articles is implausible in English (the implausibility shift is probably due to the very different nature of the article system in the two languages mostly due to slavic interference but this is only a conjecture).
In any case if the (definite article) reading is allowed an (indefinite article) reading must also be allowed.
if this was סבא וסבתא then it would be ok. In English "Grandpa and Grandma are cooking meat" ... Because we call them these names instead of their first names so it works. In English, Uncle and Aunt don't get the same treatment. In English it is always said with a possessive, usually "my". Unless the name is also stated. "Uncle Bob and Aunt Linda are cooking meat" We don't say "uncle is eating lunch" (although probably could be said in England with a British accent) my conclusion - since there is no sentence context, the English translation should be "my aunt and uncle are cooking meat" (aunt before uncle as a matter of normal English semantics)