how does Priestly show that Mrs Birling often pretends to be something she's not?

Mrs. Birling wants to keep family secrets hidden. This is why Sheila tells her mother that they can't keep up with "these silly pretences." Mrs. Birling is an icily impressive woman, arguably the only one of all the Birlings to almost completely resist the Inspector's attempts to make her realize her responsibilities. She hides behind her stature and public influence.