The relief you're feeling makes complete sense - living under constant comparison is exhausting and painful, and your nervous system is naturally responding to what feels like freedom from that pressure. The guilt shows how much you actually care about your sister, but it's okay to feel both relief for yourself and sadness for her situation at the same time. Your feelings don't make you a bad sibling; they make you human, and recognizing this complexity is actually a sign of your emotional maturity.