My sister is younger then me by six years. Even with the age gap, I would still say that she is my best friend. When you think about it, your little sister is just a mini, younger version of you. You were both raised by the same parents, went on the same vacations, dealt with the same hardships.
All of these experiences can really make up the character of someone, which is why you two may be so alike. Chances are that you guys may have similar likes in activities, and that favorite meal your mom makes. Your sister will be the only one that knows what its like to grow up in the same household you did. She will know the extent of craziness that your parents are, she will know the little rules of the home, and what your family is like. She will be the one that you go to to gossip and talk about things with. The only one that is obligated to stay by your side throughout life.
Having a sister is like being able to have a slumber party every night, even if you don't share the same room. She will be the one that you will cry to when you break up with your boyfriend, the one that will understand that your parents are being unfair, she will be your maid of honor at your wedding. The time you spend in search of a best friend, or someone to grow close to, you may not even realize that you already have that person in your life. The relationship you will share between you and your sister, will be that one that you always wanted.
You already have someone to take a girls trip to the mall, have a movie night, and paint nails with. Your little sister is one that you need to teach about the ways of the world. You will be the one she comes to when she is first learning how to do her make-up, when she wants to go one her first date, and what to wear on the first day of high school.
It is important that being an older sister you need to be a good role model, she will look up to you in many ways even if you do not realize it, or it may not seem like she is, but she truly is. In some ways it is like you are her second mother, its important to lead by example and show her right from wrong, before you do something think if you would want your little sister to be doing that, share your experiences with her so that she can learn from them, and so she does not have to go and make the same mistakes that you did.
Years from now when you look back on your time growing up, you will come to the realization that she knows you better then anyone as she was the one that has been there through all you had to go through. Never break a bond between you and your sister, you will regret it.