Sure, here are the sentences completed with appropriate question tags:

The weather is really bad today, **isn't it**?
We won't be late, **will we**?
John's a very good student, **isn't he**?
Mary called, **didn't she**?
Your sister works in a hotel, **doesn't she**?

Question tags are short questions added to the end of statements to confirm or check information. They usually mirror the auxiliary or modal verb used in the statement.