Hi and welcome,
You are more likely experiencing one of the more common causes even with a sibling dx, from my understanding siblings and other close relations (mother/father, brother/sister/twin) only have a slightly higher increase and the further removed genetically the less relevant any family members medical condition becomes. Keep in mind that MS isnt a genetically inheritable type of disease, so the odds are overwhelmingly higher for someone dx to be the only person in their family to be dx with MS but having said that, whilst its less common to have multiple family members dx with MS, it does still happen...
The most common causes of hand pain/tingling/cramping are cervical radiculopathy, carpal tunnel syndrome, and rheumatoid arthritis but there are lots of other causes that would get on your most likely causes list befor a neurological condition like MS so try to stay open minded at this stage.
Hope that helps a little......JJ