Knowing what I know of your family, I can only imagine how this kind of rattled everybody. However your wife feels about Nana, I do know that the love she has for her kids is what overrides all and I tend to think that unless the dynamic was seriously and dangerously dysfunctional, she'd eventually allow some sort of continued contact.
Honestly, with the husband being divorced with kids, you'd think I'd have some grand answer for you, but... All the kids grew up with divorce being a factor in their lives, two because they live in multiple homes, and one because he sees the handoff process... But honestly the two don't remember what life was like when the husband was with his ex. The marriage was over before the youngest was born. So this is just part of the process for them. Same idea for ours we have together. He only knows seeing siblings come and go from one house and he doesn't. Though, come to think of it, I'm not sure he gets the why. Like, if we were to tell him his siblings mother was once married to his father, I think it'd shock him.
The best I can say is that the discussion of new partners coming and describing their roles in relation to the kids would be extremely beneficial when done as soon as reasonably possible. I've learned the hard way by watching it unfold in the kids other home that not properly reinforcing roles in the home and not explaining the true dynamic of things in the new living/partner arrangement leaves the kids to fill in the blanks, and what they fill it with is rarely logical. And beyond that, you want to know what is stuff they fill in with their childlike assessment of what's happening and what stuff they're overhearing/being told. One is natural kid stuff and the other is an issue to be addressed with somebody else.
So basically a discussion that "one day grandpa will meet new friends, one might be a lady he marries, that changes nothing about you and how he feels about you, it changes nothing about us, you don't have to meet anybody you don't want to meet, but you do have to be respectful." Etc etc. And, given what I know about the FIL, a little refresher on boundaries with the kids and new partners isn't completely uncalled for either.
I know sooner rather than later seems jarring, but let me tell you... We are working through the fallout of what happens when one springs it on kids... Its not great. Rather introduce the idea quickly while reinforcing you will protect her in the transition than Wait to deal with it when it happens. The "I'm not as important anymore because I'm part of the old family and the new family is all that counts now" struggle for kids is *very* real.