Dear Sis--
I couldn't be more proud of you. I'm proud of this giant leap you're taking in life & I'm proud of the woman you've grown to be. You've come from being the annoying little brat sister to an incredibly intelligent, determined and strong adult. And now, you're off to the next journey in life-- one that will not only be fun and exciting, but one that will change your life and lead you to grow more than you ever have before.  
It's going to be extremely tough not having you around. More than a sister, you're such an incredible friend. I'm going to miss our time together-- especially this winter when I have to go ice skating at the city park without you. I will always cherish our memories, and will be ready to make more when you return. 
I know you're scared, but you're not alone. I may be 5937 miles away-- but I am always here for you. I may be grumpy, brain dead and swinging moods-- but I would drop everything in the world to be there for you and help however I can. Never be afraid to ask me to talk, ask for advice or literally anything. 
I can't wait to hear about your adventures, view your photos, and see how you grow & who you become. I believe in you more than you know. 
I couldn't be more happy for you. You'll always continue to make me proud.
Brother <3