I know you're looking for advice from other moms in the same situation, but I thought I could offer some insight into the other side of the story. My sister was 12, and I had just turned 14 when my little brother was born.
From the time my mom told us she was pregnant, we were thrilled. We had spent years asking Santa for a baby brother, and finally, we got one.
And for the most part, things were great. We loved to help take care of him and play with him, which helped my parents a lot, and now, we're the cool older sisters he gets to hang out with.
There are a couple things I would suggest you keep in mind, that made a difference to my sister and I.
1. Make sure you spend extra time with each of your older kids. It doesn't even have to be both parents, but just something to remind them that they haven't been forgotten in the chaos with the new baby. We would get to go to a movie with dad, or lunch with mom once in awhile, without my brother.
2. Don't use the term "built in babysitter". The terminology bugged me. What did work, though, was there was a difference some of the babysitting we did. If it was necessary, chore stuff, we were expected to help out and watch him. Fore example, if dad had to work on a saturday, and mom needed to go grocery shopping, it was expected that we help out and watch him. On the other hand, if my parents wanted to go to a movie, we were asked if we wanted to babysit, and they would pay us (as if we weren't there, they'd still have to get a sitter). Worked out alright, though we did resent it at times. Just teenage stuff, looking back. I mean, we didn want to do the dishes, either.
3. Talk to them about dealing with stupid, nosy people. I cannot tell you how many times people came up to me to tell me how cute "my son" was. It seemed more likely to people that I had a child at 14, than that my mom had an age gap between children. My sister and I got my brother tshirts with variations of "I'm the little brother" on them, but he couldn't always wear them.
I would just be open and honest with them, about what you expect, what you don't expect (for example, poopy diapers weren't something my parents generally considered in our job description). They were the parents, and while we helped, it was a point that it wasn't our responsibility all the time.
If you want to talk more, just email me--i can let you know more about what that situation meant for me.
Overall though, its been wonderful!