And what a massive big shock it must be for you both. You're not being ungrateful at all, you're just being sensible and honest about it all, that's all. Like you said, you had both finally agreed that there will be no more babies. I believe things happen for a reason, and I guess you're meant to have one more baby.
Massive for your sis as well. It must be hard, but there really isn't anything that can be done now.