birth order problem

Why is it so difficult for an oldest child to treat a youngest child like anything but the baby? My youngest sister is 11 years younger than me and she is quite an adult but I still think of her as my littlest sister (reads the baby)! She lives on her own and has for over a year and has been married over a year as well. She is in the Army and quite a competent adult. See, I know this in my head but my heart still wants to nurture her and baby her and help her make all of her decisions... I've got to get over it!