Learning to say Good Bye (cancer sucks!)

Kai will be 4 on Monday. She's growing up so fast. She is learning to ride her big wheel, she can finally walk the dog with him tipping her over.
She starts 4K end of next month.
I love everything new she is discovering and doing but I wish there were somethings I could protect her from as well.

She is a very blessed little girl. She has Grandparents and Great Grandparents that love her very much.
I only had one Great Grandma growing up and she passed away when I was 5. I hardly knew her. Her mind had left long before I was born and she lived in KY while we were in WI.

Unfortunately she has already encountered Death a couple times but she was too little to understand what was going on or why everyone was sad.
My closest cousin passed away from Uterine Cancer that spread to her kidneys and liver last October and Kai's God Father's dad passed away last August from Pneumonia.

I know she is understanding better about death and loss with the recent miscarriage but that isn't going to help with whats around the corner.

My paternal Grandfather is dying from brain cancer.
He almost beat it once but it returned to an inoperable location and I think his body is just wore out. He is 88 years old.

This is my favorite picture of them. Kai is 1. I took this right after his diagnoses.

He is rapidly getting worse. He is almost completely blind and deaf. He was hard of hearing already but now he can't really hear even with his hearing aids.
From what I've been told he's lost a lot of weight and is constantly cold. He told my Dad the last time they talked that he feels he doesn't have much time left.
Kai loves her Great Grandpa and I have no idea how to tell her what is going on. She caught me crying the other day and I told her my heart was sad because Great Grandpa is really sick. She seemed to understand that. It broke my heart how sad she looked as she told HB that her Great Grandfather was really sick.
She thinks he's going to get better though.

I hate this. I hate watching him waste away from the strong man that he was.
I am so proud of him. He is a WWII vet that fought in Normandy. He beat Prostate cancer! He made it through losing my Grandma to Stomach cancer.
I really really f*cking HATE cancer.
How did you explain it to your kids when they lost a close family  member?
Any advice is welcome and would be really helpful.