Why does Rose love Oliver in Charles Dickens' Oliver Twist?

Rose Maylie, née Fleming, has been adopted. In reality, she is a sister to Oliver Twist's mother, Agnes Fleming. She comes to love Oliver after she learns of his tragic past, to which she may be more sympathetic, having been an orphan herself.
In Chapter XXX, after Oliver is discovered the day following the robbery attempt, the sensitive Miss Rose sees the sweet child, who has been shot, laid upon a bed, and she exclaims,

"But, can you-oh! can you really believe that this delicate boy has been the voluntary associate of the worst outcasts of society?"
"He cannot be hardened in vice,...It is impossible."

Rose and her foster mother Mrs. Maylie, who rescued Rose as a girl in Wales, believe that it is environment which has negatively affected Oliver's life, rather than his innate nature. Added to this, Rose probably has a certain tenderness in her heart for Oliver since she herself was orphaned as a child. In Chapter XXXII, after a grateful Oliver tries to thank her, Rose tells him,

...if you only take half the trouble to please us, that you promise now, you will make me very happy, indeed."

She adds that her aunt's rescuing him from "such sad misery as you have described" is an "unspeakable pleasure" to her. Moreover, knowing that Oliver is sincerely grateful and attached to her delights her, Rose tells Oliver.
Later, in Chapter XLI, Oliver reports to Rose that he has seen Mr. Brownlow, so she takes him to the gentleman's house. This visit results in the clarification of Oliver's and Rose's identities. For, Mr. Brownlow was a good friend of Mr. Edwin Leeford, who fell in love with Agnes Fleming, the older sister of Rose and the mother of Oliver. While Oliver says he must still think of Rose as his sister, this news certainly strengthens their bond of love as they now know that they truly are related.