On there own VM-BB, TV or phone are expensive in comparison to deals offered elsewhere, especially BB, but as solcuerda says add a phone line to the BB package and the price drops substantially, even more so if you add the TV as well.
As regards special offers for new customers, VM are not alone in doing this, it seems most companies now offer these introductory prices to "set the hook" like it or not this practice is widespread nowadays.
It may well be worth the OP's sister having a look at some of the BB and phone deals that VM offer, I'm sure she could save a few pounds.
