Mrs Potter was Mr Dursley's wives sister, but thankfully they weren't very close, in fact they hadn't met for several years, Mr Dursley often pretended his wife didn't have a sister because both Mrs Potter and her good-for-nothing husband were as unordinary as it was possible to be. Mr Dursley believed they had a son but he honestly didn't care, since Petunia was happy to pretend they didn't exist he himself tried not to worry too much about the family of freaks.