Sentence:  And just as `` Laurie '' Lawrence was first attracted to bright Jo March , who found him immature by her high standards , and then had to content himself with her younger sister Amy , so Joe Jastrow , who had also been writing Henrietta before he came to Johns Hopkins , had to content himself with her younger <WSD>sister</WSD> , pretty Rachel . 
Meaning of the word marked with <WSD> tags:  a female person who has the same parents as another person