I'm sorry this whole family system must be very confusing for her too. I really think the best way is to have her father take care of this, including threatening to call authorities or getting her to help personally. It will look vindictive if you do so, even though your heart would be in the right place.

You say she has always been aggressive, so I doubt any one call will solve her problems.
However, it doesn't sound like a good environment, from what you describe, for the 3 year old. I hope you can convince him to act.