I don't really have any advice, except that this issue was addressed in Dear Abby yesterday, except the other way around. The person wanted to throw a shower for her twin sister, but was told by her husband's family that it would not be appropriate or something.
I thought it was interesting that "Abby's" response was out of the etiquette books, and it said that it is NOT right for a mother or sister to throw the shower, but perfectly acceptable for anyone else, including MIL, SIL, aunt, cousins, or friends.
I know that doesn't help your question, but I just thought I'd share. I, personally, would hope that my sisters would hold the shower for me if/when I was pregnant, and I would do the same for them. As a friend, I think you are only "obligated" to do what you want - and that includes making the shower as small or large as you feel comfortable.