Yeah... plus for this theory to be true, I'd have to accept that my Grandma is covering up the fact that he had one or two types of cancer with absolutely no mention of it to anyone else in the family, not even the slightest hint towards it. That's irrational
I have actually had thoughts in the past along the lines of "What if my Granddad died with undiagnosed bowel cancer and it's hereditary now?" so I wouldn't put it past my deep subconcious to create a dream-world where he actually had got it