First, I hope your sister's surgery goes well. Second, you're not coming across as selfish. When FFIL was diagnosed with leukemia last year, wedding stuff was on the back of my mind. When he passed away in September, it was on the back of my mind then as well. It's an important event and you want to be able to share it with those you love.
I'm not sure if this would work, but can anything be arranged through a cruise to get her to Cuba?