What does Fela mean when she reports to Minou that the sisters "must finally be at rest," since they won't come to possess her?

Fela is the Mirabals' maid, and she has continued to work for Dede. All along, she's believed that she can commune with the three dead sisters, and she tells Minou what they say. What Minou means in the quote is that Fela has told her the sister's won't come to her (possess her), so they must finally be at rest. There's nothing more for them to say; they're at peace.