The answer can be found when you compare Lily at the beginning and how she is at the end.
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One way in which Lily changes from the beginning to the end of the book is the way in which she views motherhood. In the beginning of the novel, Lily associates the idea of "mother" only with a legal and biological connection between a woman and her child. She displays this definition when she dreams of Rosaleen adopting her and becoming her "real mother." But Lily's relationship with her biological mother is based on memories and uncertainty. Lily spends the majority of her childhood attempting to put together the missing pieces in her mother's life. Such curiosity drives Lily to travel to Tiburon. Lily experiences feelings of anger, pity, and grief when she learns the true story of her own mother.
Along the way, Lily also recognizes Our Lady in Chains as the mother of all, including her. She looks to Mary as a mother who brings about the inner strength inside Lily.
Finally, Lily is able to connect the term "mother" to the eight women in Tiburon who have pledged their love, time, and resources to ensure that she has the best life possible. Now, a mother in her mind is someone who takes up the role of a mother.