The family history thing is to spot markers.
Usually it is applicable if the family member was under 65 when the condition developed. One family member is nothing to worry about. Many is.
Since it is your grandfather then I can't see them getting too nervous about it. If it was your father and he was aged 35 when the conditions developed then they make take a closer look.
With regards diabetes, so long as you don't have it now and you keep your weight in check then the chances are very slim that you'll get it. Glaucoma is often a secondary effect of diabetes. (Other causes as well).
Have a scratch around Google and learn more about the conditions and you'll find you have nothing to worry about.