Amy,
I also have a nineteen year old daughter, and a seventeen year old. My biggest fear when I was diagnosed was not being there for her her senior prom and graduation. I made it to both!
My daughters have been a wonderful support for me. I have photos matted and framed of daffodils that were blooming the day I got home from the hospital that my oldest did for her photography class. This year for Mother's Day she ordered Livestrong bracelets for all us and my coworkers that supported me through the last year.
The seventeen year old has become much more cuddly, agreeable, and I am "Mommy" again instead of "MOM!" or "MOTHER" in an angry tone. All of these little things have meant so much. They are both extremely busy and we can go all day without seeing each other, but they always call me if we don't do a face to face "I love you, have a great day."
Your attitude will help your father immensely. Stay positive for him, and lots of hugs and kisses are always in order! Do stuff for your mom without her asking, dishes or laundry or vacuuming. It all helps.
You and your family are in my prayers. Your dad is lucky to have you!
Sincerely,
Lisa