Did you know, Pakistan's Western borders have remained secure for better part of our history, just on the basis of this brotherhood! Maybe you didn't know this little gem of info.
Not entirely and not too that secure either. Many a times the PAF has had to scramble to attack Afghan intruders trying to claim territory. Tactics including low level buzzing to live fire were used to send them back. And the ones who reported these intrusions were tribesmen themselves who were harassed at the least and attacked and looted at the worst.
I am both Ta'ajubed and Surprised that you ignore these incidents.( Unless off course one has attended one too many King Zahir Afghan events in Peshawar and were enamoured with the gorgeous and relatively scantily clad ladies present there).