I Am Concerned About The Relationship Between Our 11 Year

I am concerned about the relationship between our 11 year old grandson and 7 year old granddaughter. He cannot keep his hands off of her. He touches her inappropriately and she says it's ok because she is little. They are living in an incredibly toxic relationship where there Mother and Father are going through an atypical divorce. The mother is sadly a woman with a severe history of drug abuse and now is an alcoholic. Her response to the children has been, you are bother and sister and that makes it ok. She unfortunately sleeps with both of them, in all states of undress. The father believes he needs to wait until the divorce is final before he can have any impact. Additionally, his social maturity is truly lacking and he appears to remain in denial regarding the impact this behavior has now and will in the future. I have suggested he request court ordered psychiatric evaluations as part of the ruling and have even considered reporting them to an appropriate department for further investigation. My husband (his son, another issue) has said to wait until the divorce is final in order not cause any more delays. Any Suggestions would be helpful. Thank you