Originally Posted By: BranShea

A picture of my sister's. The s looks like a leftover from his.
"My sister his picture". But then my sister is a her. Then I do understand you can't write or say " my sister'r picture".
You could say though: "my brother's picture = " my brother his picture"

This is a commonly held misunderstanding of the origin of the 's possesive marker in English. It really comes from one of the most common OE inflexional endings for the genitive singular.