"she had no other relations than two aunts, maiden sisters of Mr. Spenlow"

Respectable young women were expected to remain living at the home of their parents until marriage. Following the death of their parents, if sisters remained unmarried, it was common for them to reside together. One literary example of this may be found in Deborah and Matilda Jenkyns in  Cranford (1851) by Elizabeth Gaskell.

Because it was considered somewhat inappropriate for a young women to live alone, many such spinsters would provide an orphaned young woman, usually a relation but sometimes the daughter of a good friend, with a respectable residence.

In the case of Dora Spenlow, her aunts provide her with both a respectable home and an official guardianship.