You give a clue to the problem with the comment that the biting occurs when the dog is sleeping with your sister and her husband.
This exhibits a basic misunderstanding of dog behavior. A dog will bite you when ever it feels it is your equal and will bite you more and more as time goes on if uncorrected immediately.
I believe you have already let it go on too long.
When a dog shares the sleeping space (I don’t mean the same room, I mean the covers and the bed) it is elevated to equal status. This is a nice liberal ‘we’re all equal, hearts and flowers’ kind of BS.
The problem is a dog is a pack animal in it’s DNA and it cannot be anything else. A pack animal needs to know it’s level in the pack to be happy. If a dog is allowed to be ‘equal’ with it’s owner then in the dogs mind it is Alpha. The Alpha corrects behavior it finds offensive by biting as hard or thoroughly as it’s current mood demands.
I will say this as nicely as I can; your sister and her husband have condemned this dog to death. Their misunderstanding of the doberman psyche has caused them to become subservient to this animal and it will not relinquish it’s Alpha position without a serious and vicious fight.
A Doberman is a fighting animal (a war and protection animal) from hundreds of years of breeding. The dog can behave no other way if allowed to be Alpha.
Do I say all Dobermans are bad dogs? Of course not, they can be loving and great protective family members but they must never be Alpha in the family.
It is possible this can be corrected if you can contact a ‘tough dog’ trainer and by that I do not mean some sit-stay local. It will be expensive and your sister will not like how it is handled and furthermore she will not like how she (and her husband) have to be with the dog from that second on with no letup.
The dog will watch every second it is awake for a chance to elevate itself in the pack. Bad things will happen if you slip and allow that.
Probably the kindest thing you can do for that dog is to give it to the tough dog trainer or kill it before you and your loved ones are seriously hurt (although I think you already have been by the bites you describe).
BTW, I have had and trained Akitas so I am sharing hard won information, not something I read in a book.
Excellent point about the bed.
Crates training sounds cruel but it is fine if you do not neglect them..
Most will find that if a dog is crate trained, they will sleep in that crate even if it is in the bedroom and the crate is open.
Mine may sleep at the foot of the bed occasionally.