NOBODY knows why things happen. We can only live thru them and either allow them to make us stronger, or let them to bring us down. Truth be told, NOBODY knows why god does the things that he does or why he gives us the trials that he gives us except god! Your sister, is being very judgemental. That is one of the things that our father in heaven does not want us to do! You are a smart, fun, compassionate person. Dont let this get you down. Just get ready to be there for your sister when she does need support. MS is a terrible disease and she WILL need you, because unfortunately, it will not always be kind to her.
A good friend will help you move, a GREAT friend will help you move a body . . . Let me know if I need to bring a shovel!!!