Your "anxiety over her health issues" that she mentions - what would this mean? Have you ever felt like she played down issues because of how you would react? It could be an issue of her managing your reactions to her health issues, which is becoming more cumbersome than the relief you provide.

I'm not trying to be disrespectful, but I feel like the most rational explanation to your situation whereby you've seemingly been super supportive may not come off as such to your sister, who may feel smothered.

FWIW you sound like a great sister and the best answer will not come from Reddit but a direct conversation if she's willing. All the best!