Why has Zeena agreed to take Mattie into her house (direct,attributed quote), and what is revealed about her character?

From the text:

Mattie Silver came from Stamford, and when she entered the Fromes' household to act as her cousin Zeena's aid it was thought best, as she came without pay, not to let her feel too sharp a contrast between the life she had left and the isolation of a Starkfield farm. But for this—as Frome sardonically reflected—it would hardly have occurred to Zeena to take any thought for the girl's amusement.

From this quote, we can infer that Zeena is sickly, cheap (no pay at all?), selfish and thoughtless. Zeena almost seems to think she's doing the girl a favor.