Explaining death to a toddler

I’m not sure if anyone has any advice or maybe recommendations of resources that could help me. My mother in law was recently diagnosed with stage 4 lung cancer. She is now in the hospital on a very high level flow of oxygen, and my husband and father in law are going up today to discuss hospice options. They haven’t told us how much time they estimate she has left. I’m due with our daughter March 20th, and we’re really hoping she makes it to see her.

My son just turned 3, and he loves his grandma. I have no idea how to explain to him what’s happening, or how I’ll explain when she passes. If you have any suggestions, I would really appreciate it.