In my understanding -
"Weathercock" is in relation to the ground (an object). {compass heading}
"Slip" is in relation to air flow (relative airflow). {unbalanced flight}
Weathercocking is not relative to the ground, it's relative to the wind! If you enter a slip, there is a weathercocking tendency that is prevented by using opposite rudder to remain in the slip.
Dihedral is used to help bring the aircraft back to level flight if a slip is encountered.