When a loved one's actions hurt the ones who raised us

Assalamu alaikum. My younger sister embraced Islam a while back, and alhamdulillah for her guidance. That's not the issue at all. What's tearing me apart is how she's treating our grandparents. We all live in the home they built, but for years she's acted like they don't exist. They are the most kind, generous people. My grandmother, may Allah reward her, has spent her life in service to family and charity. She taught me compassion from a young age. The problem isn't her deen. It's that she plans to just... wait for them to pass away, avoiding them until then. She said it so casually, and it broke my heart. Our grandparents are elderly and need company. I spend my days with them-cooking, helping with medicine, keeping them company-trying to fill the gap she leaves, even though I have my own health struggles. They ask about her constantly. They message her, invite her over, and even send gifts and money through me. She accepts it but never shows gratitude, not even a simple 'jazakAllah khair'. The worst is when she makes promises to them. She'll agree to come for a birthday dinner, knowing full well she'll fake an illness at the last minute. Then I have to cover for her, watching my grandmother cry, blaming herself for talking too much. I have to lie and deliver 'get well' remedies for a sickness that doesn't exist. She won't visit them when they're ill or in the hospital. I see their health declining, and their only wish is to see their youngest grandchild. I've told her she doesn't have to discuss her deen if she's not ready, that she can just spend time with them at home. But she doesn't listen. I can't bear my grandmother's tears or her self-blame. How do I make my sister see the pain she's causing to the people who love her most? Please, any advice would be a mercy. Jazakum Allah khairan.