Bill's eyes
Kate's brother
Tom's and Bill's grandmother
My uncle's and my aunt's son
Their grandfathers books
Mark's hair is long.
My sisters hair is short.
Tom's friend is 18 old.
Bill's and Mark's hair is short.
His brother nose is big.
Если мы говорим о чьих то вещах мы приписываем 's к личным местоимениям, или существительным, которые имеют данный предмет.