Dont know how to handle this. Please dont judge

My oldest child is in care(age 15). Behavioural difficulties - this has been a few years now and everyone is accepting of this fact, including oldest child.Contact has been brilliant and steady for a year now. Problems have been arising, only i have been too ashamed to admit it to anyone due to how far we have come.It is mostly between my oldest and my 9 year old. My 9 year old just needs to breathe to set my oldest off. Normal-ish sibling thing... This was being managed well and my oldest was seeing that when he put effort in to be nice, his brother was calmer and more pleasant to be around.Only now its at the point if my 9 year old enters the room, my 15 year old will look at him with disgust and hate, speaks to him like crap and then claims 9 year old starts deliberately annoying him which provokes his reactions. When in reality, 9 year old is reacting to what i have just described. 9 year old craves the acceptance of his older brother, but he will not even entertain the fact.It has resulted in what i can only describe as bullying - it is constant and repetetive whenever they are around each other. It is slowly escalating though. So much so he threatened to throw a knife at my 9 year old. He wouldnt - he didnt have a knife, nor the actual intention of doing so. Only my 9 year old does not know that! He has in the past thrown scissors at me and another sibling.Since this has been deteriorating for months now, rather than improving - the only thing i can see to do is to stop contact between the two of them. Only this would mean 15 year old would only be here 2 to 3 times a week, rather than 3 to 4. Overnights would not change.This feels like i am letting my 15 year old down though, but if i continue to allow contact then i am letting my 9 year old down.There are another two siblings. 8 and 13. He has no issue with them, but sometimes tries to get the 13 year old to join in on the picking at 9 year old.It took a lot of work to regroup the household after my oldest. Mentally, i could not take it if it went back to how it was. It took a hell of a lot to reign my 13 year old back from trying to copy the behaviour. The last year massive progress has been made and i fear its all going to come undone. via /r/Parenting http://bit.ly/2F2VjS6