http://www.cardiacandvascular.com/patient_ed/aaa.htm gives a lot of information about aneurysms.
Trauma can cause an aneurysm! That said you mentioned his brother had one
aneurysms can be hereditary!
I don't think aneurysm comes from trauma, I think it is pressure from the blood on a part of a blood vessel that is thinner than the surrounding area. Possibly trauma could increase the blood pressure, but that would be the only connection, as I understand aneurysm.