Oh Crissy! *hugs*
I'm so sorry for you and your family. I've been through this with my father-in-law who had terminal lung cancer, and we knew for months that he was going to die.
As his journey progressed and he was truly unwell, we had this very discussion with him. To be honest, I don't remember the exact words but it went something like, "We love you so much and know how hard this is for you. We want peace for you and the pain to go away. So we just want you to know that we'll all be okay. We'll always be here for mom. We'll always be here for each other. It's okay for you to go. We'll miss you terribly. We'll think of you every day."
As I write this, I'm tearing up thinking about it and it was more than 6 years ago. We cried a lot during this pretty short conversation and my father-in-law didn't respond much other than nodding. Over the next two weeks he deteriorated dramatically and during times when we knew he was in severe pain or not "with it" we'd remind him "It's okay, dad. We love you and will miss you, but it's okay. We WILL be okay."
It was so hard. Again, I'm so sorry. Please keep us updated.