Pretty big age gap over here too!

My DD is 7, my stepson is 12, and baby-to-be is due in two weeks.
DD has gradually gotten more and more excited and is really happy to be a big sister at our house too (she has a baby brother at her dad's that just turned 1). I think she will be a great help. Baby is due the week she gets out of school for summer vacation. She hasn't agreed to change any diapers, but she does promise to "get me stuff". I'm excited to enjoy a relaxing two months off of school with her and getting used to being a mommy to a newborn again.
I'm not sure what my stepson will think - 12 is a funny age, and he's a boy (so no motherly instincts there). I know he will love it once baby Caleb is old enough to mimic him (and say things like "poop").

This time around I do plan to savor his babyhood more. I am in a MUCH, MUCH, MUCH more supportive relationship, with a very invested hubby and daddy. I also will be working way less. All bonuses for this little guy!
The only thing I see sad about the big age gap is that the kids rarely are playmates for each other. Every once in awhile DD and stepDS do "play", but its getting less often as he gets older. It will be the same thing for DD and baby-DS. I suspect they will be friends as adults and really enjoy each other (he will love her to entertain him, I'm sure), but never "playmates" like siblings only 2-3 years apart.
I'll try to find this post and update a couple weeks after baby arrives and we see how things really go!
ETA: As for the baby shower - if someone offers to give you one, I say go for it! I ended up with a rather small one, which is what I wanted, but because of its size, many wondered for weeks if I was having a larger one. For us its the first baby of a 2nd marriage for us both, so special in other ways.