Iunderstand your anger on behalf of your siblings and what your dad did to your sister was terribly wrong and inexcusable. But, it sounds like they are adults which means it is their responsibilty to decide what kind of relationship they want to have with your dad. Your dad can only continue to cause them pain and saddness if they let him. If you allow your brother to vent to you about your dad thats your fault. All you have to do is tell your brother you wont listen to any more whining about your dad. Its called establishing a boundary.
I think you are too emeshed in these other relationships and are taking on a burden that is really not yours. The best way to help your siblings now is by modeling for them what it means to be an emotionally healthy adult.