Why would you recommend that they work on being able to get together with the step-grandfather? Instead you should urge this father and his sister to try to get their mother into counseling to find out why she thinks it's okay to have this abuser around her grandkids. Taking the kids around this man is telling them that his actions were okay and their safety doesn't matter. Grandma can come around again when she leaves her husband. She absolutely needs to take a side in this. There is zero defense against this kind of behavior.