Being a big sister is a huge responsibility because you have to set a good example. Your little sister is always watching you and wants to do everything you do. She looks up to you and wants to be like you. That's why it's important to be a good role model.
Our five year old is very playful with her little sister. They fight like normal siblings, but they are very close to each other. Being a big sister also means helping your little sister when she needs it. Our five year old is always willing to help our three year old. It could be something as simple as helping her with a zipper or helping her play a game on the computer. Both of our girls like to help their baby sister. If she cries they'll give her a toy or her pacifier to make her feel better.
Being a big sister does not mean you should boss them around. You should always consider their feelings because even though you're older, you're equal. Give your little sister unconditional love and avoid being a bad influence. Respect her and keep in mind that she mimics you because she admires you. Remember, friends come and go, but sisters are forever.