I was 12 when my little brother was born. My parents always made sure my brother looked up to me and I took care of him so we were always close. I'm like a caretaker-friend.
I think our age difference was actually still close enough that we actually share similar taste in music, book and games (I'm pretty sure I influenced him in a lot of these) and I feel that I can still give relevant advice (compared to my parents). I think the nice thing with the gap is that he generally listened to me, and I felt somewhat responsible for him. I think I grew up a lot being the older sister, and my bro's the primary reason I want children.
The only advice I would give is in the transition period; your older one is used to being the only child and a baby disrupts a life they've known for 13 years so they might dislike the change. As much as I loved my bro I was still annoyed that my family did nothing fun ever in the first few years. Not to mention those are the awkward and confusing years to begin with. I distinctly remember my mom's hormones going out of control and feeling really confused on why she was yelling at me. I kind of wished there would be some warning or explanation of what was happening. So I would advise a talk with your older kid on what to expect and explaining why.
And of course like PPs have said, the older kids can act as part time babysitters and help out a lot, but I think it'd be good to give a lot of positive reinforcements of this and also to encourage your younger kid to be very appreciative of this. Looking back, I definitely think I missed out on a lot of things because I had to be a babysitter, but because my bro was so great and always treated me like the most important person in the world, I still treasure those times. (I do sometimes feel a little resentful of my parents for making me give up what I wanted to do but that's another topic)
Also btw for both my college apps and my bro's college apps, we wrote about each other and how the other person shaped our lives so, I'd say our relationship is pretty special
Good luck