why didn't Sophie want to share the story of meeting Danny casey with jansie?

Sophie considered her meeting with Danny Casey which she had told only Geoff to be a secret between the two of them. It was a Geoff thing and not a Jansie thing which means that it was meant to be something special just between the siblings. It was to be a secret between the two of them and not to be shared with Jansie because she was a tattle-tale. Jansie did not understand the delicacy of situations, so she would tell the whole neighbourhood about it. Sophie was losing touch with her brother as he was drifting apart, this secret was supposed to rejuvenate the bond between the two because there are some things which are scared between siblings. It preserved the sanctity of a brother-sister bond by drawing them closer.