At first I thought you were speaking about your mom's behaviour in the past tense, but now it sounds like it's ongoing which is a little different. Your mom shouldn't have continued to favour you after your health had returned. Or at least not to the point that your sister felt rejected for it.
But again, this isn't your fault, and parents aren't perfect. Your sister is obviously jealous and feels left out and it's easier to blame you than your mother.
I have to ask, how do you feel about your mom being closer to you than your sister?