Sheila begins to believe that Eva is the girl she had sacked form Milwards.

From the text:

Sheila: (distressed) I went to the manager at Milwards and I told him that if they didn't get rid of that girl, I’d never go near the place again and I’d persuade mother to close our account with them.

Inspector: And why did you do that?

Sheila: Because I was in a furious temper.

Inspector: And what had this girl done to make you lose your temper.

Sheila: When I was looking at myself in the mirror I caught sight of her smiling at the assistant, and I was furious with her. I'd been in a bad temper anyhow.