I would suspect that with a relatively new and unusual breed such as the Sphinx, there won’t be any rescues available for a long time to come.
Several cat breeds are considered to be less allergenic than others. The Devon and Cornish Rex cats have fewer hairs in their coat and so retain less of the allergen Fel d 1 on the coat. The Siberian Cat is supposed to produce relatively little of the allergen Fel d 1.
In addition, females produce less of the allergen than males, and neutered males produce less than intact males.
You can read more about it here.
There is a Siberian Cat Club in the UK. Someone there might know about rescues. In the US there is a Siberian Cat Rescue Group but I can’t find anything in the UK. Again, this is a relatively newly introduced breed in both the UK and the US so there may not be many available to rescue.
You may have better luck with one of the Rex breeds. This site lists the rescue organizations in the UK, and one of the entries is for the Rex Cat Club , which offers a re-homing service for Rex cats. You can call their Welfare Officer at Tel: 01306 631538 and put yourself on the list.
Question: does your step-mother live with you? If not, you might simply keep a room where the cat cannot enter and which you keep scrupulously clean that would be used when she visits. If she does live with you, then you might have to settle for admiring a friend’s cat.