I would say something like this:
"Hey [Sister's Name], I just wanted to take a moment to tell you how much it meant to me to be there for you during this tough time. I know how hard it was to go through the loss, and I was really glad to be able to support you. Whether it was just sitting together in silence or sharing memories of [Person's Name], being there for you was important to me. You don’t have to go through this alone, and I’m always here for you. Remember, it’s okay to feel whatever you’re feeling, and I’ll walk this path with you as long as you need me."
This kind of response reassures her that she is not alone, acknowledges her feelings, and emphasizes the importance of family support during difficult times.