Isn't сестри a plural form?
I don't have sisters wasn't accepted as an answer. Instead it said I don't have a sister.
сестри́ -> Genitive of сестра (singular)
се́стри -> Nominative plural
У мене немає сестри -> I don't have a sister.
У мене немає сестер -> I don't have sisters.
сестри in this context is the genitive singular for сестра.
In Ukrainian, with adverbs of quantity, for example - much (багато), мало (little) and none (немає) you use the genitive case.
But of course, in English it is more natural to say "I don't have any sisters."
Should be pronounced мЕне, not менЕ because it is preceded by a preposition
Duolingo should surely accept "I don't have any sisters," right? If not, then how would one say that in Ukrainian to differentiate it from "У мене немає сестри"?
Немає requires geneative after it. If you were to say "I have no sisters", it would be "у мене немає сестер"
This sentence could be an answer to "How's your sister?".