You can pick from an endless variety of tattoo designs, but some people prefer to get a tattoo of significant meaning. One of the closest bonds is the bond of sisterhood, and many sister like to get tattoos to represent that love and bond they share together.
One of the most popular sister tattoos are the celtic style ones. There is also the chinese symbol that means sister, which is a very popular choice as well. Another idea, is to simply pick something that you and your sister both really like. As an example, if you both like roses you can either pick out a rose design or even better go to a tattoo parlor and let them design one for you. All you need is an idea of what you would like and they can sketch it up for you.