cream tabby boy/girl sibs, white with blue tip boy/girl sibs, and blue boy/girl sibs.
So you've got three flavours there Barney :)
There's a gene that they call the dilution modifier that says "whatever colour you were going to be, make it much paler", which would be involved here.
Blue is actually "black + dilution", cream is "orange + dilution".
So without the dilution modifier you could think of those cats as being:
black (the blue ones)
ginger tabby (the cream stripey)
white with black tips (white with blue tips)
You wouldn't think of that as being SO unusual.
In the same way that there's a separate gene saying "and-white" to get white splotches, there's also one that says "be white/pale in the warm parts of the body, and only show colour at the tips".
I'd be sceptical of anyone saying that any combination was "impossible", because there are so many different genes involved and they interact in various ways.
I am probably massively oversimplifying here, and even with that it's complicated!