As a cancer survivor myself (luckily, mine is not terminal, but that could change any moment).......the only thing you want is to have people with you to remind you of how sweet life is.
When I was deep in chemo, we traveled to Chicago for a 3rd opinion on my treatment. I was talking to a lady in the waiting room who was there with a friend who had terminal liver cancer. I had the epiphany that I'd not want anything more than to spend every waking moment with the people most important to me if mine was terminal.
Jen P. had a nice recommendation with the memory book. But, I'd be cautious that what is cathartic for the rest of us may be harmful to someone with a terminal illness.
So, my best advice is to ask her. Be honest that you'd really like to do something to celebrate her life and give her something special. Ask what she'd like. My guess is that spending time with her in some capacity is probably the first thought, and it will mean so incredibly much. It's OK to ask.
What kind of cancer has she fought all these years? 15 years is an amazing accomplishment.
My other thought is to celebrate her by contacting an organization like LIVESTRONG who often runs stories on people with amazing stories and immortalizing her fight for all the people who may face her diagnosis down the road.
My cousin will be 36 on Monday. He was just diagnosed with Stage III Esophageal cancer. It's survival rate at 5 years is ~20%. His daughter is not yet 1 month old. All he wants is to see his daughter's first birthday at this point. People like him can be so encouraged by her story of fighting for so long. It helped me after my diagnosis with Hodgkins (my kids were 11 weeks and 2 years) - 5 years seemed like an amazing gift at that time. I'm 2 years out and apparently OK. But, anything can happen, and I try to celebrate the time I have on this Earth as much as possible in small ways each day.
Good luck to your Aunt and your family as you prepare for her passing.