One way to hold this word: the ending -hood turns a person into a state, so sisterhood can be pictured as being sisters. Start with two girls in one family and the closeness between them. Then let that same closeness stretch over women who are not related at all but stand together for the same thing. Then let it stretch once more, to a house of women who choose to live as sisters every day. The family tie is the picture; the other uses borrow it.
From the word’s history
Wiktionary records sisterhood as Middle English susterhede, that is sister plus -hood, with the ending later respelled. The ending is what does the work: it names the state of being the thing, which is exactly the picture the modern senses share.
字典記載 sisterhood 來自中古英語 susterhede，也就是 sister 加上 -hood，只是字尾後來換了寫法。關鍵在字尾：-hood 把「姊妹」變成「當姊妹的狀態」，這正好就是這個字今天各個用法共用的畫面。
Wiktionary entry for sisterhood, supplied in this batch's evidence packet: From Middle English susterhede; equivalent to sister + -hood (with a modification in the suffix).
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