Parent of an Adult adopted child with attachment disorder

I am new to this - I am the parent of three adopted children, ages 24, 23 and 20. The oldest and youngest children bonded great with us and we enjoy wonderful relationships with them. But the middle one, failed to bond, though we did not realize this attachment disorder had a name until now. He was a difficult baby, did not want to be held, rocked in his crib when we first got him, did not want any boundaries as a child growing up and by age 14, he would not listen to anyone. He was drinking, smoking pot, did poorly in school, sneaking out of the house, turned us into CPS if we told him he had to finish his school work, etc. He has no feelings toward us,his parents, or his siblings other than to "say" they are his brother and sister. He has told every friend that we have that we made him eat only cold cereal when everyone else had hot meals, etc. that he never got nice gifts for Christmas - basically smearing us in every way. He easily shows more consideration and feelings toward people who do not reciprocate back to him any love at all. He has difficult relationships almost abusive with women. We had family counseling because he was on probation for physically abusing me, his mom, when he did not get his way - pulling out phone lines and breaking them when I tried to call 911 - thrown my furniture off my decks laughing and daring me to call 911. The counselor worked with us for about a year and he finally sat us down and told us that there was nothing wrong with our parenting, that it was obvious that we loved him, but that he was a child who did not want to be parented and so he suggested that we just bide our time with him, and as long as we were not in danger physically from him, to have him live at home until he reached 18. True to the counselor's words our son moved out the day he turned 18. He has struggled ever since - finally getting a felony DUI at age 20 injuring two men in his car that had to be air flighted to hospitals. He is an alcoholic and addicted to pot. We have shown him love through all of this but he does not acknowledge it at all. When he was released from jail the morning after the DUI, we were there waiting for him -we hugged him and told him we loved him. He appeared to genuinely be hurting inside and we were there for every court appearance. But he continued to drink and lived with a man who enabled him in this area and so any positive help we tried to offer was seen as interference on our part by our son and this man who took him. Our son surrounds himself with people who enable him and allow him to have no accountability for his behavior. We suspect our son is bi-polar with some schitzophrenia because his birth mom is schitsophrenic and his half birth sister is bi polar with schitzophrenia. His relationships with his adult siblings is very hard - he treats his sister very badly and she adores him but tolerates his taking advantage of her and the mental abuse he dishes out to her, but then our daughter struggles with the hurt she feels. I have tried to talk to her and let her know that this is the way her brother is and she needs to be aware of that in every situation so she does not get hurt. But this has come to a head because our son rear ended his sister and caused her whip lash and damage to her car which he refuses to acknowledge any responsibility for the accident, and even slapped her head around laughing when he made her cry cause her neck hurt. He has made verbal threats to her for calling his car insurance company to file a claim and threats to us. When is it time to just let go of him and move on. I feel it is going to be like this forever and he is still violating and playing the legal system with his DUI offense - violating probation by drinking and driving. The threat of prison time is being held over his head with the law but they have yet to follow thru. He has violated probation at least 10 times in two years. We only see him about once in every 3 months or so even though he lives 40 minutes from us and he is often in our community, but we see him on his terms - we are glad to see him but know he comes back for the connection not really because he loves us. We basically only feel comfortable with him for about an hour then he starts getting to us and he knows he is doing it, and we are glad when he leaves. I am still very afraid of him even though I am his mom. Is it wrong to cut ties with him under these circumstances - would we be doing him more harm and should we just tolerate him - we do not expect change on his part because he thinks he is "normal." He stood by and watched and knew one of his friends was raping his sister after his friend put GHB in her drink and did nothing to stop it - but his friend got away with it because my daughter waited to report it until we got home 18hours later because her brother threatened her if she called police - evidence was on her clothes which police found but they could not press charges because of the length of time since the act. our son to this day is still friends with the man that raped his sister. I am 57 and exhausted emotionally - I need a break. Am I wrong for feeling this way?