I'm not sure if this question is as much about cake as it is about kids and baby shower etiquette, but .. whatever. Here's the thing:
I've been hired to do a cake for a baby shower for a girl I work with. I'm also invited to the shower as a guest. I happen to know that the new mom also has an 7 or 8-year old daughter - I'm guessing on the age.. she's about thaaat big (You all can see me, right? See how high I'm holding up my hand? Like, boob-height?). Anyway, I was thinking that such a fuss will be made over the new baby, that Older Daughter might feel left out.
What if I did a really small cake just for her? Or some cupcakes? With some kind of theme that's appealing to young girls? Would that be .. out of line? Inappropriate? I've really got no radar for this kind of thing (and no kids, and questionable social skills, obviously... )
Whaddaya think? I'd do it for free, just 'cause.. Hmm.. but then I wonder if the people who ordered the shower cake would be mad, 'cause I'm sure as shootin' charging them. And what do young girls like? Princess theme still fairly safe?
Myself, I would ask the host before bringing something for the older sister as they may already have that covered.
I have an upcoming similar situation with my daugher-in-law who is expecting my second grandchild. She told me she wants to have a "big sister shower" and I had never heard of such a thing. Of course, I had never heard of diaper showers either until recently...guess I'm showing my age here.
Anyway, that's just my opinion. I think they need to know what you are planning on bringing, even if it is extra and at no charge.