In what way does priestly make the relationship between shiela and gerald such an important part of the play? (an inspector calls)?

The whole thing revolves around the fact that the family are gathered for Sheila and Gerald's engagement party. Although the couple are caught up in the intrigue - Sheila has a particular role as commentator - her attitude changes through the play and she sees the failings of the various people involved and helps vocalise Priestly's views. Gerald also introduces the idea that the inspector might not be all he seems - suggesting if not revealing a fact which has probably become clear to the audience, but which Priestley clearly does not want us to miss.