First of all, in my family, even though I'm the stepmom, both of my kids just consider me another parent. My step son was 11 when we started discussing having a baby. He was not too keen on it and actually forbade me. We had the discussion of what is his place in the family - he got to have input, but did not get to dictate whether or not our family was added to. I got pregnant and he was very upset. He didn't voice it to me initially because he wanted to spare my feelings. I told him that as his parent I didn't get to choose when I listened to what he had to say. I told him that it was ok that he wasn't ok with it, but that didn't change the fact that a baby was coming, so how can we work through it. Part of his issue was that mom lives in FL and he never got enough time with Dad because of the distance. (Mom moved from KS to FL at the divorce.) Additionally, Mom had introduced a lot of changes in his life that, for the most part, he was not happy with, so change was a bad thing to him. Peanut was born in October and that January he decided to come live with us here instead of in FL with Mom. When he went to grandparents and mom's to visit, he said he missed Peanut most of all. I don't think he can imagine life w/o Peanut now.
Guess what I'm saying is, been there, although mine was more about the older than the younger and this was my first baby. Mostly, it's about bringing it up before you're pregnant so you can work through the issues before you're hormonal. Also, we talked a lot about how we're a family, all of us. Another person doesn't mean there is less to go around. Like when your husband and his son joined your family - the baby is just a new member of the family.
And middle child isn't so bad. The distance between everyone is sufficient that they are all in their own stages at any given time. They won't even be in high school at the same time.
Btw - 13 years between hubby and me. I married him and got two kids in the deal: my daughter is almost 16, my son is 13. Now Peanut joins in at nearly 10 mo.