Suspected Emotional, Verbal and now Physical abuse of a family members children

Hi, Just after some advice from people who have experienced similar things or maybe come from a child protection field!!
I am one of 4 siblings who live across 2 states. One lives in the hometown with our parents. We are concerned about the well-being of two of the three children who are with this sibling in the home town. Our sibling has anger management issues which have been apparent since a teenager. Over the years we(siblings) have been concerned about the verbal and emotional abuse the two older children have at the hands of their parents. They are often called Bitches and cows, told that they are welcome to move out but the parents will not be responsible for paying for their education, over the years they have been told that they were never wanted, and this goes on and is relatively constant. Today I found out that my sibling hit the eldest child in the mouth causing a swollen lip. My gut instinct is that I need to notify the child protection department, however my concerns are my elderly parents live in the same town and the backlash that they may get at the hands of my sibling is concerning. The other siblings are between 2 and 4 hours drive away so being their in a hurry is virtually impossible. I have today spoken to the eldest child affected giving them my mobile number and another siblings number, with the authorization to call reverse charges from a pay phone if needed. I have also told them that I am happy to have them move in with me. I suggested that they speak to a teacher or welfare at school in a hope that someone can give them some coping strategies.
My gut instinct is that I need to be advocate for these kids, but the repercussions of doing this is what is stopping me/us. I have spoken to my parents about making a notification and although they think that is what is needed,(one parent was a mandated report of child abuse in their profession) they are also concerned about the repercussions and also that the children will be stopped from having all contact with extended family which would be an absolute disaster from the children.
If you have been through something similar or maybe come from a child protection field and can offer any advice, we would appreciate it.