I was out, I missed all the fun.
There is no £50 activation for Sky Broadband Unlimited, either for migrants or new customers. So, assuming no contact was made to Sky by your Sister about Sky Fibre or Lite, this appears to be an error by Sky.
A key point quoted by BatBoy was "We will move you to the nearest equivalent or better Sky package wherever possible".
There was never a 2 GB capped product on O2, so this product can be considered worse. Lite carries a £60 fee which has been waived for you. The £3 price on O2 was a heavily discounted price as a result of a retention discount.
So, as has been said, result for you. But the only thing your Sister was really entitled to was Sky Broadband Unlimited without the £50 setup charge. I do not accept your accusation of "deviousness" on Sky's part as it appears nothing more than an error.
Oliver.