It might. She's not trying to get the sister to stop praying or to change her beliefs. (I mean, she shouldn't be trying that. She might have been trying that previously, which would be why it didn't work.) I can totally understand the temptation for LW to argue that her sister should stop praying for her, that she believes God hates her, that she does not want to go to heaven. Those are all invitations for dispute, for a religious person.

"Stop praying IN MY PRESENCE," is a much simpler and more direct request. It does not invite religious dispute. But why not? The sister might dispute it, but it becomes social, or family dispute, not theological dispute.
"But why not?"/"Because it upsets me."
"But I want Him to help you!"/ "That's between you and Him. Your conversations with Him upset me. Don't have them here."
"But it's really important for you to invite Him into your life now that you're so sick!"/ "I want you in my life when I'm so sick, but if you can't refrain from praying for the length of a visit [or phone call] I won't talk to you."

The sister doesn't have to change any of her beliefs about God to abide by her sister's wishes. Just as adult siblings can have an argument around "dad abused me and I never want to talk about him again" and "he was always nice to us and we never saw anything abusive," and still support the "never want to talk about him again."