Live in forida.
Niece has lived with us for 18 years.
Niece is 21 and has a 3 year daughter.
She is dis-respectful, and at time full of rage.
I feel she may try to harm my wife.

How can I get a niece with a child out of my house?

{removed}

Have her formally evicted.

First where is this, in some Asian nations you just throw her out, in the US or Canada you will have to give her a formal notice to move, and if she does not have her evicted though the courts.

Does she pay rent? Was there an agreement to pay rent?

If so, and she doesn't you can give her a 3-day pay or quit notice. Otherwise you give her a 15 day notice to vacate. If she doesn't then you go to court for an eviction order.

That she is a relative does not matter so I've moved your question from Family Law.

First where is this, Florida