A portrait tattoo is a life-like rendering of a photograph. You can choose almost any picture of your little sister, at any age level, but bear in mind that portrait tattoos are often extremely detailed and will need a large flat surface of your body to be portrayed adequately. Portraits are also one of the most difficult styles so be sure to employ an experienced artist and don't be afraid to examine several artists' portfolios before choosing the right person for the job.
A script tattoo honoring your little sister can be anything from her name to a lengthy quote that can be attributed to her. There are hundreds if not thousands of fonts and styles to choose from when selecting a scripted tattoo, some more extravagant than others, requiring more surface area. But an even more personal way of choosing a script style would be to have your sister write her name or the quote on a piece of paper and have that image tattooed.
Choose an item that is significant to you and your sister. It can be anything -- a type of flower you both like or a jar of pennies you once saved together as children -- as long as it has a shared significance. The particular object will dictate the detail and how much skin will be needed; you can decide whether black and gray or color is appropriate. A seemingly random object can be a thoughtful tattoo, as it's meaning will only be apparent to you and your sister.
A tattoo of your sister's initials and birth date can be added to any of the previous ideas or represented on its own. Again, you can choose from a variety of fonts and styles, or create the script on your own. Another way of introducing a line of letters and numbers is to tattoo an object that already has alphanumeric script, such as a license plate or bar code, and instead, insert your sister's initials and birth date.