Sorry to hear about your issues Jeremy, I can understand your concern. However, don't jump to conclusions just yet; self-diagnosis is a hazardess thing. There may be many factors that your doctor will consider in making a diagnosis. For one thing, MS is not truely a heridetary disease. I´m sorry to hear about your sister, MS is truely a terrible disease, a close friend of my wife has it as well. Thet fact that your sister has it doesn´t give a substantial bigger chance for having it though.
Do you smoke? If so, your finger & toe issues could also be caused by a condition called Buerger's disease. There are also several auto-immune disorders that could cause similar problems, or certain types of vasculitis, or diabetes, etc etc etc.
In other words; there are numerous possibilities, try not to jump to conclusions and worry a lot by making your own diagnosis. It may be all not as bad as you think. I hope it is not!
