14 year olds struggle with siblings.

[audley]

I have a 14 year old and a preteen. They have been best buddies until the last few months. The age gap suddenly seems vast and they can’t get on unsupervised.
Am I being reasonable telling the younger one to keep his distance for now, or AIBU and should be expecting more from the 14 year old?

Both, surely? They're old enough to understand the issues and rise above their disagreements.