Huge problem, need advice.

I need your help with something that has been bothering me.  This is something that could tear a family apart and I need some advice.

Here is some history on the situation...  My BIL and his ex wife have a little girl (A) who is almost 4 years old.  A's parents got a divorce last year because her Dad was VERY verbally abusive towards her Mom.  We aren't sure if there was any other kind of abuse but the verbal stuff would happen in front of everyone almost daily.  A's Dad immediately got into a relationship with another woman and is also verbally abusive towards her and her daughter who has CP due to being born 3 months premature.  He has also been verbally abusive towards A since before her 1st birthday.  A's parents share custody of her and when she is supposed to be with her Dad she 99% of the time ends up at her grandparents house (my IL's).  He pawns her off on them like she is a burden and they in turn coddle her to make the situation better.  A's mom is a great mom but likes to party and often leaves her with her grandparents on her weekends also.

Fast forward to yesterday evening.  We went over to IL's house to visit and A was there along with her Dad and his gf and her daughter.  Everything was going great until my DD needed to use the restroom and A followed her in there.  We've told A many times that she is not allowed to go in the bathroom when our DD is going potty but she still follows her in there.  None of us had noticed that she had followed DD in there until A's Dad went looking for her and found her in there (DD was telling A that she needed to get out so she could use the restroom, door was still open).  Instead of listening A ran screaming and crying to her grandma who began to coddle her.  A's Dad stood over her on the couch and began yelling at her "Shut up or I will give you a reason to cry!".  When that didn't work he yelled at her "Get up and go in the room so I can beat your a$$!" among many other things.  She wouldn't move the whole time he yelled at her, my DW tried to get him to stop but he wouldn't and no one else would say anything to him.  As soon as DD was done in the restroom DW suggested that we leave.  As we left DW apologized to her Mom saying that we would have liked to stay longer but...  BIL then started yelling at DW that we didn't have to be there if we didn't like it.  We left.

I am so upset about that whole situation and I don't know what to do.  I want to call the county and report the abuse so she can be placed in a safe home (her Mom is aware of his verbal abuse and still allows visitations).  If I called the county than the family would know it was myself or DW that called and would flip out.  We can't take A because it would cause a huge rift in the family.  My IL's won't take a stand against BIL because he is their miracle baby (almost died at birth, coddled him ever since).  No one is willing to do anything to make a change for A, they like things the way they are.  I am also concerned about D, BIL's gf's daughter, he is also verbally abusive towards her and she can't do anything about it (2 years old with CP, adjusted age is about 1 year).

What should I do?