Sounds an awful and frustrating situation you're in there, Colin.
If the service is "Not yet live", this is terminology commonly applied to the fact the 'port' has not been switched on. Obviously, Openreach Frames Engineers will have had to run jumper-wires to reconnect your sister to the new ISP's equipment, but if there is no juice to go down it ??!!
You could try Openreach (doubt they'll entertain you though, as your 'contract' is with Tesco), to enquire as to whether the 'Frames Activity' has been 'Commed off' on CSS. Don't worry about the acronyms, they'll know exactly what they mean. This basically ensures Openreach have been allocated, and have fulfilled their, task obligation. In laymans terms, the wires have been run to the correct equipment.
My only fear, if it isn't down to the fact the port hasn't been switched on, is that your sisters line has been allocated a complete new routing from the Exchange to the DP. It used to happen extremely regularly, but not so much these days.
Going from the terminology used though, it sounds like a port issue, on the face of it.