I hope your sister is recovering. What a terribly stressful time for you.
I can't really shed any light on the timing of your relapse. If you have a faith, it may well be that God was looking out for your family, and not giving you more than you could handle at one time. I know the faith I share with my husband has helped us to see many blessings in the midst of difficult trials.
The one thing I would encourage you to look into would be the vascular connection to your MS. Since your sister had a stroke, there may be familial situations, like cerebral bloodflow, clotting factors, hypercoagulated blood--that could be compounding your MS. Make sure to exercise, don't smoke, eat a heart healthy diet, reduce stress. You and your sister can help each other in this pursuit. Here's the program I created for my husband, who did have a vascular componant to his MS. http://ccsvi.org/index.php/helping-myse ... ial-health