Sora's mother lives in Hokkaido and the class trip has brought them to Hokkaido, so I am assuming her mother has made her return. If I recall correctly, the mother's permission for Sora to stay behind was temporary and conditional. And the matter between them hasn't been fully resolved yet.
Considering that the mother started a new family and seemed pretty glad to have her out of the way, I don't really see why she would get involved unless she absolutely needs to.