Martin and Katia - welcome to the forum. I'm sorry that it's under such sad circumstances.
I think that certainly the first approach is to the Breed Council to establish if the breeder is a regulated licensed breeder. I think I would also be drawing it to the attention of the RSPCA, because if the breeder is selling indiscriminately then it should be investigated. Any reputable breeder will appreciate that.
In terms of monetary compensation (always an ugly subject, but pertinent, as pedigrees are never cheap to take on) you could commence a claim against the breeder under the Small Claims process in England and Wales (assuming the breeder's England/Wales based). This applies to any claim with an overall monetary value less than £10,000.
It's relatively inexpensive - you pay an issue fee to the court based on the value of your overall loss (for example the cost of the two kittens, any transport costs paid, and any verifiable veterinarian costs) but you should be aware that even where you successfully obtain a Judgment, it may not be easy to obtain compensation from the breeder without having to take some form of enforcement action.
Whilst you can recover a degree of fixed costs for your outlays, it is a time consuming and often frustrating process. If you instruct a solicitor to act on your behalf for example, you would not recover all of the expense that you would pay to a Solicitor, and could end up being out of pocket even where you technically "win" your claim.
You can bring the claim yourself using the Money Claims Online service https://www.gov.uk/make-court-claim-for-money/overview
Your claim would be based on the Sale of Goods Act 1979.
This may be of use:http://paag.org.uk/buying-a-pet/report/
The Small Claims Courts also offer a free medation service to its users, where once proceedings have been issued should a Defendant (what they call the person against whom the claim has been brought)) file a Dispute (called a "Defence") challenging the claim. The Mediator is an independent, who will attempt to get the parties to address any areas of disagreement, and reach a legally binding resolution to the dispute.
I'm so sorry that the cats and you and Katia have had to undergo such an appalling experience.