Question

1-The weather is very nice (change exclamatory sentenced)​

Answer 1

wow! so nice weather. :) :)

Answer 2

Answer:

Weather" is not a "count noun": you cannot have one weather, two weathers ... many weathers" except in an unusual construction. So every sentence that includes "a weather" without a following noun is a mistake in ordinary English. When "weather" is used as an adjective--"a weather phenomenon," "a weather problem," "a weather alert"--the article is OK.

Perhaps you might answer your niece's question with a question: "what is a weather"? If she's a native speaker, she may get the point. Or tell her it's like "water," and ask whether she drinks a water when she's thirsty.