"Uncle is not ill."
Translation:मौसा बीमार नहीं हैं।
Mausa beemar nahi also should be accepted as there's no need to add hai for negative sentences
Nope. While you don't usually need a है for negative sentences, it is necessary to add one here because otherwise the sentence wouldn't have a verb and would be incomplete.
I would like to know how frequently "mausa" is used and by whom. My Indian husband and his family use chacha (father's brother) and mama (mother's brother) or they will simply refer to an older man as "uncle". I am just curious to know who uses "mausa".