Custody and Visitation Issues: How Can a Father Get Custody

My question involves a child custody case from the State of: St. Croix, virgin islands.

I am a 26 aunt who is concerned about her nieces who are 5 and 6. Their mom is a 22 year old who is about to give birth to her fourth child (only the first two are my brothers'). Ever since she broke up with my brother and is dating someone else I feel like she neglects my nieces. My brother and our family provides for my nieces. Every well my sister goes for the kids so that she can wash and comb their hair for school. If she doesn't (from ongoing observation), my nieces hair does not get washed or comb. Not so big of an issue I take it. Recently my sister observed a cut on my niece ear when she inquired about where it came from, my niece said her mom was beating her and she got a cut on her ear. Most recently my sis was bathing her a felt what assumed to be a lump/knot on her forehead. Once again my sister inquired and my niece proceeded to say she got beat because she didn't understand her homework. I have called human services and before arrival to my nieces mother house they called her. How do they expect to find anything if the person under investigation gets a heads up that they are coming to the house. She has recently denied us and their father visitation (not by law but because she said so) because she has recently found out that we have been taking pictures of the cuts and bruises. She is about to deliver her fourh child and my nieces are staying by Her mother who does not treat them well instead of their own father. I am not sure what todo. Please ell