It's not stretching the truth to say that Calinda is the most mysterious character in the book. While she and Delphine both conceal their identities, Delphine talks so much that we find out a great deal about her, anyway. On the flip side, Calinda is largely silent. She's a mystery until Delphine reveals that Calinda is not her slave but her sister; she just happens to have darker skin.
We definitely get the sense that Calinda, for all her visions, is the more practical of the two sisters. She seems to realize that they don't have a never-ending cash flow and sets up a small business selling pralines to the riverboats. When Mama loses her mind over Noah, Calinda is the one who takes care of business, making plans for Tilly and Delphine to travel while she stays behind caring for Mama and Cass.
We find that Calinda has just as much, if not more, to lose than Delphine with the loss of their New Orleans way of life. First, her nickname comes from the fact that she's the best Calinda dancer in the city. In fact, she's famous for it:
The room held its breath when Calinda rose at last from her chair [...] Of course she was a dancer. Why hadn't I seen it the first time I ever laid eyes on her? She was a dancer in every step, every turn of her head. (8.47)
New Orleans culture, then, is a source of pride and strength for Calinda. She's also a seeress, however, an ability that's highly valued in New Orleans. Again, then, we see Calinda bolstered by where she comes from. Her seeress skills didn't just help her back home, though; they also solidify her bond with Cass:
Calinda was the sister Cass needed [...] Her and Calinda spoke a language I was deaf to, a language of prophecies and cures, of visions and the medicines waiting out in the timber to heal the afflicted. They spoke in tongues foreign to me, even when they weren't speaking at all. (6.18)
Cass is in dire need of this connection, shoring up Calinda's value beyond the confines of New Orleans. Though Delphine is the one who stays with the Pruitt family long-term, there's no question that Calinda makes significant contributions to their well-being, and not just through her herbal cures and delicious recipes.