What are the chances of one family getting hit with osteosarcoma twice in less than a year?

Pretty good if your dogs are of a certain age and certain breeds.
Traditionally, we have placed St. Bernards, Rottweilers and Great Danes at the top of the list, while recognizing that all large breed dogs are at greater risk for this cancer of the bone than smaller breeds.
A study published in the Journal of the American Veterinary Medical Association October 1, 2007, however, postulated that perhaps Greyhounds were overlooked victims of this disease because of the small

numbers among the pet population. That situation is, of course, changing because of the large numbers of retired racing greyhounds being adopted.
Still, we know that simply “being big” is a substantial predisposing factor.
Add that to being the most popular breed in the USA and you get the next dog on the list, the Golden retriever. Veterinarians see a lost of osteosarcoma (OSA) in Goldens simply because of the sheer numbers in the pet dog population.
Sara, Catherine’s “sister,” just had her seventh birthday last month.
I just examined her in mid-July.
Her owners just noticed the lump three days ago, and it’s already clearly visible and somewhat large. The worst case scenario is that her tumor might take a rapid and aggressive course, like Catherine’s did.
We certainly hope not, but we will keep you informed.