why do you think the author added the part about paul mother in the novel?

I believe that Paul's mother (actually his family) return to the novel in order to show us exactly how much Paul has changed. He feels separated from his family because of what he has experienced. In this section (I hope you're referring to Chapter 6), we can see how far removed Paul is from the things around him by his refusal and inability to confront his feelings about his mother's illness.