No excuse for someone of 72 to say that! I am a person of faith, not a million miles off her age, suffered 7 years of infertility and pregnancy loss heartbreak myself and I cringed at the words you report.
Is she actually part of a regular congregation and involved in activities beyond Sunday worship - a house group or prayer circle perhaps - or is she just 'religious' in the sense of parroting thoughtless clichés?
If she is active in a church, could you ask her to seek advice from her fellowship and its leaders to get some ideas on what she could say that would be helpful rather than alienating? There are certain to be people there who understand first-hand what you are going through. They can be blunter with her than you feel able to be!
If she is a nominal Christian with no church links, I'd be inclined to think you could politely say, "Nan, your comments about God's plan are just making me feel as if what we're going through is punishment. It's very upsetting. Could you please just say you're praying for us instead?"
If she agrees and sticks to it, fine. If she agrees and re-offends, remind her the first time and if she carries on, you could just calmly say, "Nan, you're doing it again, so I'm going to say goodbye and hang up now. I love you." Click!
If she actually tells you she thinks you ARE being punished and there must be unconfessed sin or something in your life, you are entitled to hang up and go NC forever!
If babies only came to people who were perfect, there wouldn't be many of us walking the earth, would there?
If I made a list of all the crass, cruel, ignorant, thoughtless and painful things people - some of them kind and well meaning, some of them loving friends and family members - said to us on our journey it would fill a book so sad nobody would read it.
Yet when we succeeded, those same people were quick to tell us they 'always knew' we would have a happy ending! 🙄