I really like Momandlovingit's response.
I think your granddaughter needs her own bed, and you can go in and read to her, and have PJ time, until she's drowsy - but let her sleep in her own bed. If she needs to regress a little to feel secure right now - totally understandable. Let her have that. But do it in her bed.
Once she's settled - then have the occasional sleepover fun night with Grandma. Let it be about fun, watch a movie, etc.
I don't see this routine as ending well. She'll wake up, check on Grandma, it can result in anxiety, it's from fear - which never ends well. Kids need adults to set boundaries and say "You will be absolutely fine in your bed and it's the best place for you to get your sleep". They need the adult to say that. They don't need their two adults disagreeing on what's best for them. This will cause confusion and upset them. Agree with mom.
I am sorry for loss and I hope you fully recover from your operation :)
Added: I also like B's suggestion of therapy - I think that's a great idea because support comes in many ways, and you've all been through a lot. Kids often just need to talk to an adult who has not been through any of it. They just open up like a flower and let it all out - which is all kids need sometimes.