...to his brother...
We can legalistically wrangle about that. Should we go with the spirit or the letter of the command? When Jesus defined ‘neighbor’ He painted with a broad brush. But those intent on saying to others, ‘You fool,’ will likely find a loophole. Me, I am taking no chances. The repercussions are way too serious, and from the very mouth of Jesus Christ at that.
Bottom line, no thanks; the only time I use the expression ‘you fool,’ is when one or another of my dogs does something extraordinarily stupid. Otherwise, the words do not leave my lips or my keyboard. Better safe than sorry.