Your concerns about your niece are certainly valid. It does sound like there is a lack of supervision and role-modeling in this girl's home.

What about calling the CPS hotline and line out what you are seeing to the caseworker without giving names and see what they recommend before making an actual report?

Maybe the best thing you have to offer your niece is a solid, loving environment she can visit and an adult she can trust if she needs someone to turn to.