Playing devil's advocate: You supported her sister. How well did you know her sister?
The reciprocal would be her supporting your brother. How well does she know your brother plus throw in that we are now dealing with the opposite gender.
I was wonderning this too.
OP, I have to say that personally, I'd find it a little strange if my sibling had cancer, and a friend of mine was sending them weekly cards and gifts. To me, that's a little extreme. Unless of course, my friend was also friends with my sibling. And if the reverse happened and my friend's brother had cancer, I wouldn't feel comfortable in sending him cards and gifts unless I knew him well too.
I'm also going to agree.
What you did for her sister was very kind and sweet and considerate. However, it way, way beyond anything I would ever do for a friend's sister. And, to be honest, it's way beyond what I would do for a good friend with non terminal cancer.
[Sounds callous, but I personally have some health issues that can sound scary, but aren't-or more correctly, I don't find them alarming. I ran into someone-a friend of a friend- who has the same thing as me and it was a BIG DEAL to her and her family and her life.]
I would visit. I would bring a meal. I would donate to a group gift. I would never send weekly gifts. It's almost unfathomable to do that for a semi acquaintance who will likely get better. I don't say this to be a jerk. I think it's lovely that you did that. i admire it. But I would never do that.
[Other story. My FIL had a heart attack and a few years later a car accident. Both times, I did nothing that I recall. My husband drove out 10 hours the first time and flew 8 hours the second time. Which reminds me that I did do something. I had a free flight anywhere from an airline mess up. I gave that to my husband to book the last minute flight, but that doesn't really count as anything special. But my FIL's sister sent weekly cards.I thought that was sweet too, but never going to happen]
Anyhow, that was a long winded way of trying to get to a few points
1) What you did was sweet, but exceptional
2) Your brother is in a scary situation and the sad thing is this paralyzes people
3) This is your best friend. Talk to her and tell you what you specifically need.
4) Tailor that to what she can give (money, time, compassion,expertise)