How and what do i tell my god son that his father died

So my brother in law has been sick over the last three years with a brain tumor. He has been getting worse and worse over the last months. Wasn't able to walk for the last 3 month and basically lost one body function after the other. He is now in the hospital and will probably die in the next hours. My Sister is at home with her new born (5 days old) and my god son (4 years old). She is devastated and is barley able to care for herself. Thats why i am planning to spend as much time with my god son as possible and give him some support. He knows that his father is/was sick but has no idea that he is about die and the meaning of death. So if it happens tonight, what should i tell my god son and how. i am absolutely clueless how to handle this situation. I would greatly appreciate any help i can get! via /r/Parenting http://ift.tt/2eEFdQ5