what does sophie mean when she says this was a geoff thing not a jainse thing?

Sophie had shared her tale first and foremost with her brother Geoff. She considered him a confidante and felt that their little talks were secret. She had felt Geoff capable of keeping secrets and so called her conversation with Geoff a 'Geoff thing' because it was meant to be kept a secret from all. On the other hand, she believed Jansie to be efficient at spreading information among all and so did not want to tell her her secret as, according to her, it was not to be circulated among the masses. Thus, her sacred information was not to be treated as a 'Jansie thing' but a 'Geoff thing' and was meant to be kept hidden from all.