A lock and key tattoo is another good option for sister tattoo, as it represents how important the two sisters are for each other. Description from fabulousdesign.net. I searched for this on bing.com/images
My mom and her best friend did this in collage, different designs tho. My mom had a 1 half of a butterfly and her friend had the other half. I thought it was really pretty! Maybe I can get a pic of it on Pintrest!