Rashi on Proverbs
A rebellious brother...of a strong city There is a brother who rebels against his brother and loses a strong city, e.g. Lot against Abraham and Esau against Jacob.
Rashi on Proverbs
and the quarrels are like the bolt of a castle The quarrel between them separates them forever, like a bolt with which they lock the gates of a castle, so that no one should enter it.