There are some tattoos that are specifically designed for best friends. The sister tattoos are some of these tattoos meant for best friends.
21 Brother-Sister Tattoos For Siblings Who Are the Best of Friends I would get a 3 as the third sibling
Inspirational Ideas of Sister Tattoos - Listing
Seasonal Moon Names and symbols. Nice if you are only wanting a small tattoo, but I think I should go big or go home!
Like this placement and how dainty these are. Also cute if you're into the bff/couple tattoo thing
22 Awesome Sibling Tattoos for Brothers and Sisters - TattooBlend