TLDR at the end.
You and I are very similar. I'm now 29 and my youngest sister is 17. I left for college when she was little, but before I left she was my little buddy. I have another sister who is much closer to my age, we were in school together through high school and ended up at the same college, so our relationship was VERY different. Anyway, when I wasn't working my youngest sister and I would go do little things, sometimes we hit the mall and talk about the toys she thought were cool, we'd take a walk and grab an ice cream, we'd just go run around at the park, etc.. At her age everything you do is going to be a memory for you but builds a foundation for her. She probably won't remember it but it'll shape what she thinks is normal and acceptable.
Once I left for school it was kind of hard for me, because she grew up while I wasn't around. When I came home I made sure to spend one on one time with her. I asked questions about what she was learning or doing or watching. During the semester I'd write her letters (at her level) asking questions about what she was doing in school etc.. trying to maintain an active part in her life.
Once I was done with school and making money it changed what I could do. I wasn't able to move back to my home town (very small) because I couldn't find work there so it was still from a distance and really only seeing family on holidays. I tried to stay involved... She was falling in with some troublesome kids (I asked my mom and dad stuff too) and her grades weren't great so I made a deal, straight A's I'd replace her broken iPod... until she asked if I'd do an iPad because she bought a new iPod. So I had something new to talk to her about and she had a new goal. I brought her to some shows and museums when she could visit, she liked visiting because she had freedom to whatever she wanted (within reason) as a young teen in a big city she felt pretty damn cool.
Now that she's older she texts and talks to me about things. Sometimes stuff she wouldn't talk to our mom about and I know she won't talk to our sister about. We make plans together to go out, I'm the cool brother her friends talk about. I know my place, I'm not a teenager and I don't pretend that I fit in with them. I think that's a real risk when you're so much older; you can be goofy and relate to your sibling, but trying to fit in with their friends is creepy and weird.
I know she isn't my kid (it kind of reads that way to me in part of what I'm saying) but watching her grown up has been huge for me. She's still the little girl I held when she was like 6lbs and just shit, slept, and screamed all day and night. She's the kid that wanted to hold my hand when walking, I had to comb her hair because mom was too mean, she sat and knotted up my curls making me look like princess Seraphina watching all the damn barbie movies - even if she doesn't remember that. She's the one my mom called me and told me about the winners she was bringing around and how my parents were trying to motivate her, but she was a stubborn preteen.
Being an older brother with a big age gap is a big responsibility.
Short List:
I made sure I didn't tease her too much. I wanted to make sure my sibling relationship was positive since it wasn't something we could build daily. It's tempting because teasing is normal between siblings, but when you're not around it becomes what they know you for and it isn't a good way to build a relationship when it isn't the normal, daily, dynamic.
You're a lot older and you're another man in her life, you will help shape her perception of men. Be wary of what kinds of girls you bring home. At 18 it probably won't matter but she'll take note of who you introduce to your parents and it will show up in her actions. Seriously. I brought home a young... lady that was the biggest mistake of my life. My sister had a new hairstyle and some new very familiar clothing like a week later. She used some of her expressions. Of all my indiscretions, introducing my innocent little sister to my ex was the biggest mistake of that relationship, well besides getting involved with her.
Don't split your attention between her and your phone. Seriously.
TLDR: Being a much older brother has been seriously awesome for me. You're a role model, an idol, and a friend so remember that it's a huge responsibility but really a gift. You're thinking about it so you're doing well already. Good luck and embrace it while she still worships you because, even though you're not a parent (don't act like one) she will go through a phase where everything you do and say is stupid.