AjnaT: What you call "venting" a lot of us here call the process of healing. You've come to a good spot, and I am inspired by your courage on behalf of your niece.
When you stand up and do the right thing by your niece by taking the suggestions above to protect her, one of the byproducts is that you will be furthering your own healing. There's something about doing for others what you wish somebody would have done for you that has a backward good effect on your own life; and everybody here would agree that you are worth it!
To add my own 2 cents (when did these keyboards quit having the "cents" sign?), by reporting this situation to the authorities, you will be putting your life at risk; and so you would need to do it anonymously or move where your brother and his lot cannot get to you (and it sounds like this would include your bio mother).