They will drive you insane! I once asked my sister to stop poking me so she started poking "around me" instead. Cue the eye roll, please.
It is NEVER their fault. My sister could literally set the house on fire and it would somehow be my fault. I don't know how she does it, but she's pretty good at it.
You will ALWAYS be the unpaid babysitter. Over summer breaks I would have to plan where she would have to go if I wanted to hang out with my friends. When I started driving I had to pick her up from school too.
You experience everything before she does. You can pass on your knowledge/wisdom to her and she might actually listen because you aren't your parents.
She'll back you up when you're fighting with your parents. She might not be much help but at least it's more of an even fight.
You can use "I can't. I have to babysit my sister" excuse whenever you don't want to leave your house.
She will always see you as a role model. I can't lie, it's a nice feeling knowing that someone will always see you as the best you can be.
She's always down to hang out. She will literally drop anything for her cool big sister!
Most importantly, when you go off to college across the country, she will want to facetime you ALL the time and it is actually the sweetest thing. (This is kind of a bittersweet thing because you also feel like the worst human, for being busy when she wants to talk, but oh well.)