Colon cancer starting on the exterior of the colon is not the norm but it has been mentioned here before, it does happen. Given a family history and the few signs you describe, not to mention that loving cousin, by all means get the CT scan.
A CT scan is a piece of cake. The only thing that may throw a wrench into your comfort would be if you suffer from severe claustophobia, otherwise you will have to undergo a bowel prep and drink a large glass of contrast or have an enema containing the contrast solution. You may have an IV as well. Not much different in these respects from a colonoscopy except that you get to remain on your back