lisah - I am very sorry that your cat has leukemia
I am sending vibes that the antibiotics deal with her immediate health issues, and that she gains weight and stays healthy for a long time to come. You should discuss with your vet whether there are any supplements you can add to her food to boost her weakened immune system, such as L-Lysine, although there may be others. 1.8 kilos is very small, my 7 month old Oriental kitten is now 3.3 kilos - although Orientals are a little bigger than Sphynxes, and he is male which generally get larger.
I don't think that you can really say that there's an 'average' weight in general for cats of no particular breed. There is such a difference between the average weights of purebreds of different breeds, and a mixed breed cat could have all sorts of influences in its ancestry. I think it's more important to pay attention to the shape of the cat in relation to its frame size.