Where is the line between inappropriate parenting and abuse?

Hi reddit, I have a parenting problem that's making me very uncomfortable. This is a throwaway account, for obvious reasons. In short: I am worried that my step-father's inappropriate behavior towards my sister has become something more sinister, but I'm not sure who to ask for help or even if anyone would believe me. My mother is going through a messy divorce with my step-father, so I'm afraid that my good intentions will be seen as a custody agenda. I'm the oldest child, in my 20's, and I moved out of the family home five years ago because my step-father was verbally and psychologically abusive and manipulative. My half-sister is 8 years old, and my mother tells me that she is incapable of sleeping in her own bed because her father insists on sleeping with her. This, on top of the obsession he has with her and the way she is afraid of being away from him for even a day or two makes me sick to my stomach and I want to make sure that she is not being sexually abused or otherwise damaged. Who can I approach about my concerns that will take them seriously? We have a CFI (child and family investigator) whom I've left a message for, but since I'm not actually living in the house I don't know who will listen to me. Thanks!