when a siberian cat has a kitten with say a siamese or something, is the cat still hypoallergenic? or does it depend on the other breed?

oh no, they would not be hypoallergenic. Once you mix breeds you bring in some characteristics of the other breed, and that includes causes for sensitivities. I'm severely allergic to Siamese - and even one eighth will bother me. Think of it this way, if you are allergic to peanuts, you obviously won't eat peanuts, but if you eat something that has a few chopped peanuts in it, you will still have a reaction, but possibly not as severe. You may or may not have an immediate reaction but continued exposure would likely increase sensitivity.

There are no truly hypoallergenic cats. The allergens are in the cat's saliva and all cats groom themselves. I have mild cat allergies and I stay away from long haired cats. You can also build up a resistance to cats you have repeated exposure to.

Well first off, how is this going to happen? Breeders of pedigreed cats are VERY protective of their lines and tend to sell kittens altered and never allow crossbreeding. So if someone's claiming they have "Siberian mixes" they're only greedy backyard breeders trying to make a buck. Don't believe them.