Am sorry this mystery is messing with your lives.

Some are more prone to multiple aneurysms. There are three basic factors in this order: genetics, lifestyle and defects we are born with.

What lifestyle does he lead? Atherosclerosis is often a factor for multiple, or any aneurysm of a large vessel. Poor diet, smoking, no exercise.

Genetics: I am not sure if they have testing for it or not, but if it were me I would insist on genetic testing if a second aneurysm is found. That way all your related siblings can be tested and keep closer eye on lifestyle and other factors to try to diminish chances of a fatal episode aneurysm.

Defects: Marfan's and other (many) congenital anomalies are known to cause weak arteries. For example: people born with Down's Syndrome have a higher risk for certain types of brain aneurysms. Anytime you have a birth defect that affects the heart (such as Down's) or certain muscle types, as well as connective tissue disorders - say, scleroderma, you would likely find those persons would have a higher risk for aneurysms.

My sister died at age 42 of a femoral aneurysm. It is a fast death, the only silver lining to such a tragedy.