Can I write a note containing the f word to my niece?
That is offensive. Why would you write such a note to your niece?
She is practicing late nights on stage which should be banned
It's wrong to use such foul language towards your niece. You should be nicer to your family members.
There's no need to be nice to them.
Sure there is. And you shouldn't use foul word when talking to people not only your niece. It is offensive and not a good way to communicate.