Well, (from my own personal experience) it won't be the typical sibling realtionship, but that's not to say that they won't have a relationship at all. I am the oldest of 5 kids. I have one sister who is a year younger, then my dad got remarried when I was 16, and I have 1/2 siblings who are 17, 18 and 24 years younger than me. I was still living with my dad when my brother was born, but left for college a month after sister #2 was born, and was graduating from college when sister #3 was born. In the first year or so, my relationship with my brother was almost like he was my kid. I felt like I babysat all the time & that I was the only one who could stop him from crying. I remember his first word, his first tooth and when he started walking. But, I still felt like the older sister and remember that I swore I would buy him beer when he was 17 (I actually never did that). When I went off to college I LOVED it when my brother and sister #2 would come to visit me. All my college friends have watched them grow up too and we still talk about my brother running up and down the halls of my freshman year dorm. When I went home during those college years, I'd spend all my time with my brother and sister #2, but after college (and after my sister #3 was born), I was focusing on my own life, so went home less and less. Now, I'm 40, my brother is 23, sister #2 is 21 and sister #3 is 16.
I've had to make an effort to re-connect with each of them. Having my own kids has helped with wanting to improve my realtionship with them and of all things, Facebook has helped a ton. I think the older ones definitely look up to me as the "older sister", but the youngest sees me as just OLD. Hell, I'm the same age as most of her classmates' parents. And even more oddly, my dad & stepmom have always discussed parenting issues with me. Because they were nearly 20 years older when the "new" ones arrived, their parenting style has changed and they don't parent my younger siblings like me & sister #1 were parented. That actually was a bone of contention for many years between me and my dad.
It's a very different dynamic, but I now cherish the realtionship I have with my brother and sister #2. It's finally a reciprocal adult relationship rather than an old-enough-to-be-your-parent-but-I'm-not-your-parent relationship. Sister #3 is a work-in-progress because she's still sees me as just an old lady. I expect when she gets older it'll get better, just like it did with the other ones.
FWIW, my stepmother is also the oldest of 5 and her two youngest brothers are 18 & 21 year younger than her. Her parents felt that their "oops" baby (baby #4) needed a sibling because they anticipated that the older ones wouldn't fill the sibling role given their age gap. So, they planned baby #5.
Oh and I forgot to mention -- prepare yourselves for all the crazy looks you'll get when you are out with the new baby and your 17 year old son (without your husband). When my dad & I would go out with the babies (without my stepmom) I got death-stares from strangers unless I told them that the baby was my brother - -and then they just looked confused! LOL!