How can I handle obsessive sibling who is trying to control all aspects of our parents life? Sibling extremely unreasonable and never wrong.

Just avoid the sibling as much as you can. If nothing has ever been any different it is unlikely it ever will be. This sounds like a "take charge" person, and it sounds as though the parents "take up for this person" so leave them ALL to their own devices. Offer your help if you can be of help. Don't give opinions because it is a waste of your time and your breath. Make your own life, your own friends, and stick with those who appreciate you. You cannot ever change people. And a Type A won't even know you are trying. This sibling wants to control your parents; your parents like this sibling's control. So leave them to it and step away.

77 is not that old. Dad needs to stick up for himself. My Dad would not have put up with a child talking to him that way. My DH is easy going but not to the point his daughters would get away with telling him what to do.

Is Dad living with you? If so, ban your sibling from YOUR house. Tell him/her when he can respect you, Dad and your home then he/she can come home.

If Dad living in his own home, then he needs to stand up to this bully. So what if he blows up. Have someone there when Dad tells him that he/she is not welcome in his home if he can't respect him. If it gets out of hand, call the police and then file a restraining order. Dad is #1.

As a mental health therapist you must realize your sibling has a mental disorder that needs to be addressed. No one needs to be the brunt of her/his abuse and that is what it is.

Is this sibling in control of parent legally? As in having POA for healthcare and/or medical decisions. Is sibling executor of parents will? Does parent value sibling’s opinion and not yours? If those are all true, it’s likely there’s nothing you can do to change the situation