I want to know how I can get custody of my nieces help

Hire an attorney and file for them in court

You have to prove the mother and father unfit. You have to prove why you would be a better parent than them. You have to prove that you are able to financially support the children. You have to prove that you have adequate shelter. If you think you can prove all of that, get a lawyer. Expect this to put a big strain on your finances.