My sister is being annoying today, but usually she is nice. (We use progressive tenses for actions that are temporary)

If we are late, they will be angry. * Being late is a real possibility, so you should use the first conditional.

Use a comma before and when adding three items or more.

There are twenty-two students. We commonly used there is/are to express existence.

Don't use THE when talking about things in general.

He is not really nice-looking, and yet he has enormous charm. Use a comma before any coordinating conjunction connecting two independent clauses.

If I were a child, I would play outside. * Second conditional = If + past tense, subject + would/could/might. The second conditional is used for unreal cases.

Everyone has seen that movie.

I fell asleep while I was watching TV. *After while use past progressive.

I have not eaten anything today.

You don't need commas if "and" is used to indicate addition.

IBGE, the Brazilian Institute of Geography and Statistics, is an important institution. Don't take for granted that your readers know things.