Hi Marko,
I think if I were you I'd be trying out those 2 new kittens, and see how they fit into the family. My worry would be with Twiggy. I think with Nuda there would be no problem. But I'd be using the safe room introduction system in this case, whether it be a spare room or a large metal cage I think it would be an easier way to get them used to one another. It will allow Twiggy and Nuda to get used to their scent before they were confronted with the kittens, and it will do the same for the kittens.
My experience with this sort of thing is that one of the older cats will take on mothering one or both of the kittens, and when it comes to worrying about the claws a good pair of clippers, or a grinder used every couple weeks will help to protect Twiggy. Who knows maybe Twiggy my take on mothering the kittens.
Wouldn't that be a surprise?
Stranger things have happened.
We think you should go for it.