Is it normal to anticipate friction between teenagers and a new stepparent?

Your Grandfather is a WISE man! The chances are greater of there being problems than not! Yes, kids give their bio parents a hard time and step parents an even harder time! Weather they get along great now or not, there are bound to be problems in the future. Not that they can't be dealt with, but it would be very naive to think it's going to be a basket of roses!