Ann didn't know Sheila and the twins were coming to stay either. Perhaps she and June didn't speak as often as has been implied.
It may be that June kept all things regarding Sheila close to her chest. It's natural, when a family member is mentioned, for people to ask questions. I get the feeling that June didn't want to talk about her. According to her friends who'd known her since all their children were young, whilst the other mothers talked about their children, June said little or nothing about hers. Maybe it was simply that she didn't want to sound as if she was boasting?
Ann, as a child, had obviously been close to June and Nevill. As an adult, perhaps life circumstances got in the way. I don't think, as an adult, she was as close as she believed herself to be. On the whole, I don't think, as a family, they were particularly close.