"Eldest sister-in-law, I heard that you are ill, I'm worried. I hope you can recover soon, and I will pray for you. "
Or:
"Eldest sister-in-law, I know I can't give you strength by your side, but I want you to know that I care about you in the distance. I wish you a speedy recovery. "
The important thing is to express your concern and support, and make her feel warm and cared for.