My heart goes out to you. Losing one so young is heartbreaking.
The thing about looking for an explanation that's hard is that even if it's genetic, it may not be a complete explanation. If it's environmental, it's too tempting beat yourself up over what might have caused it. When I lost mine to hemangio, I researched everything I could about it, trying to figure out where I failed my dog ... and there were no answers. The sad truth is no one knows why so many dogs are dying of cancer.
FWIW, I recall reading (maybe in the China Study?)a theory about cancer in humans that genes that are responsible for cancer lie dormant, and may never get turned on, until the right environmental or dietary trigger activates them. The same triggers wouldn't cause cancer in a person that lacked the gene, but they would in a person with the gene. So, if that theory is right, it may be that both your oncologist and Carm are correct--- the cancer genes were there, and something turned them on. If so, a lack of cancer in line so far doesn't necessarily mean the genes aren't there in littermates or other litters, just that their dormant cancer gene hasn't been activated.
There are others here who know far more about genetics and causes of cancer than I do. I just want to offer that it may not genetics or environment, but a complex interplay between them.