(((alone in the world))) - sorry to hear about your BIL's prognosis. I remember when I was a teenager and had to visit my sisters FIL - I too felt very uncomfortable and had anxiety. Now that I'm older and had considerable deaths in the family / acquaintances, I'm more confident with how to act.

I've found its best to act normal unless you have some kind of peace to settle. For example with my mother I had some nice things to say and thank her for. We recently had a friend pass away from cancer and I treated him the same as when he was well. Only difference is, I asked if there was anything he and his wife needed.

Trust your instincts - if you feel sad, that's fine and if you don't that's fine too. I found the friend just wanted company. Death is something we'll all be touched by and in the Western culture we don't handle it as well as other cultures. I wish you strength in supporting your BIL and your sister.