People share with people in their own time. Do not fall into the trap of making it about you not being told. We did not tell the family two years for MS and six months for cancer. We worried what the reaction would be. I did not want worry or pity.
Now that you know you can have talk.
Alex
Deciding when and if to tell friends, family, or work colleagues about one's health is a very personal decision. I can understand that you feel hurt that the information reached you via a third party, but your sister may have had her own reasons for the timing of her disclosure or lack thereof. MS is not contagious, so it truly does come down to her having the right to keep it private. I'm not a fan of family secrets or third-party information myself, so I'd be inclined to simply speak with her. "Do you have MS? It hurts that you didn't tell me yourself."
As for your own health, having a first degree relative (siblings or parents) with MS increases your to about 5% relative to the .1% risk in the general US population. (citations: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3085300/ and http://www.ncbi.nlm.nih.gov/pubmed/3376997)