There can be many reasons of its knot in its stomach. Most common problems like Hernia and tumors cause this problem. Take it to vet and get its proper check up whenever you find anything like that. It is only be diagnosed exactly after taking its tests and getting its X-ray and ultrasound. It can be hernia or a tumor and requires ultrasound and X-ray. Your described condition shows that cat was having cancer in its stomach that was in its last stage.

 It is unlikely that the siblings will develop the same problem.
The only way to be sure what the trouble was would have been to have a post-mortem done. But vets charge for these.