at one time for five, and at another for nearly two years
Justine, we remember from Elizabeth's letter to Victor (I:5:4), had come to imitate the expression and demeanor of Caroline Frankenstein. Thus, having lived with the Frankenstein family for seven years, which is the age at which William was murdered, she has in some sense become a surrogate for his mother who died when William was yet an infant. Further in the paragraph, Elizabeth explicitly compares her treatment of William as "like [that of] a most affectionate mother."
This detail, which has no effect on anyone outside the family circle, would seem intended by Mary Shelley to insinuate a class bias into the court proceedings. Although Elizabeth speaks of her almost as a member of the family, she is not so seen by the judges or the populace of Geneva. Whatever the circumstance that separates her from an equality with other family members, the important issue is the very principle of separation by which she can be socially cast as a scapegoat.