Guilt!

[Duck on Wheels]

I agree with all of the above. Luckily your niece's prognosis is good, although the diagnosis sounds scary and the treatment sounds tough. Your niece and your sister will need you to be strong for a long time. That means that you need to take this trip, which you'd planned for a long time and timed to help you re-energize after a hectic work season.

Tell your sister you're worried and conflicted, but you can't change the plans and you realize you're going to need the strength you hope this trip will give you. Be sure to have travel insurance that covers an emergency return in case of sudden illness in the family (and that it includes sudden worsening of an illness diagnosed before starting the trip). Tell your sister you'll be back in a flash if you're needed, and that you'll otherwise be back on schedule, rested and ready to pitch in.

Oh, and bring back something that will really cheer everybody up, including lots of pictures.

Virtual hugs and whole flocks of butterflies to all of you.