How do I help someone grieve

My family is very tight knit- and someone just got diagnosed with late stage lung cancer. It came completely out of the blue this was the healthiest person we knew and he had a baby last year. We got the prognosis a few days ago and the doctor said there is no cure and no chance for remission. I am not as close to this person as other members in the family and I want to help everyone manage their pain as best I can. I don't know weather to act like everything is fine and we are having normal days or check in and make sure everyone is eating enough and healthy. Is there any advice to support all the people around me who are in so much pain?