Hi, someone who has lost 2 family members and a close friend to cancer here. Not a medical person so I won't touch on that other than to say be aware that whole brain radiation brings on dementia. They typically won't use it unless it is use it or you die and they count on the idea that if they are using it means that the patient doesn't have long. At least that's what I've been told by a friend who's wife has brain cancer.

My advice is to take full advantage of every minute you have with your relative. The doctors are frequently upbeat and paint a picture that is not realistic (my dad thought he had another 3 years, he was gone in 6 months. My mother in law thought she was going back to work, she was gone in 2 months. The doctors should have been more realistic).

One of my great regrets is not pushing to get all my mother in law's friends gathered for a party. She insisted that she didn't want to do it "until she was better". She was "fine", bed ridden, but alert, mostly not in pain, we could have had a party. What I should have said was "That's cool, we'll have another one. Let's do two, I'll set it up". Instead I folded and I regret it too this day, I'm a weird dude, I actually loved my mother in law, she was a cool lady.

tl;dr: go see your relative NOW. Hang out, drink some wine, tell some stories, or just hold each other.