Brothers sometimes hate each other for the most unimportant reasons, but older relatives like grandmothers rarely do. If they aren't complete morons you should think about your own life choices.

Being old doesn't mean you can't be petty. My aunt doesn't care for my sisters or me... largely because she doesn't like her sister-in-law (our mother). Add the "misfortune" of me being born before her second child by a couple months... There's no reason to project onto the OP that they must somehow need to reflect heavily on their life choices. Grandma could be a jerk just because that's how grandma is.