Jimmy is picking on Johnny in the playground. Johnny's big brother steps in and says 'Oy, Jimmy, pack it in or I will kick your arse.'

Jimmy's big brother hears about this and says 'Oy, Johnny's brother, if you try to kick my little brother's arse, I will break your leg.'

There has been no threat against Johnny's brother but it seems acceptable for him to intervene on his little brother's behalf.

Perfectly describes the situation. In more ways that one.