Best answer by Mavis1
i can think of a couple of reasons it gets cancelled so quickl.
1. She is accidently cancelling the switch without realising.
2. What meters does she have if they are prepay then she wont be able to switch.
The current supplier could block it but it seems very quick to be cancelled so i doubt it is that.
apart from that follow the woz advice and contact the team for guidance as to why its getting cancelled, your sister may have to do this as its not your account. Privacy blah blah blah and all that.