I'll second Road Fan's advice; having said that, I can speak to this a little from personal experience.
I'm only 58, but do have pretty advanced osteoarthritis (genetic predisposition). It first noticeably showed up in my thumbs/hands a few years ago.
I'm otherwise very fit for my age (largely as a result of cycling), but I do have trouble with any kind of caliper brake (road, canti, or v-brake), even using extemely good components, careful setup etc. I simply can't manage the grip effort required for any sustained braking, or for that matter trigger-shifting (repetitive motion problems) etc.
Brake quality, setup etc. can all help, of course, and may even resolve the problem completely for your sister, but in my case the eventual solution was/is (for me) discs -- problem solved, due to the very low effort required to exert braking force.