I can really relate to what you went through, though on a different time-scale. When I was a freshman in college, my brother was diagnosed with a brain tumor, and we had to go through 8 months of seizures, doctor's appointments, and the final surgery, which was a biopsy/removal of the tumor if they found out they could remove it.
I'm glad to hear you were able to be there for her. You sound a lot like my dad in emergencies; he doesn't get upset until it's all over. Like when my brother had his surgery, Dad didn't cry until the surgeon came out and said they were done and my brother was going to be fine.
My brother and I still haven't reached the point where we can be friends even 90% of the time, but we're getting there, and I think part of it was seeing him go through all of that.
I hope your sister gets well, and it makes me happy to see that she has such a loving, caring sister to look out for her.
Hopefully she values you as much as you value her.