Celia is Dorothea's younger sister, but the two of them couldn't be more different. Celia's not all that complicated of a character, and her primary purpose in the novel is probably to serve as a foil to Dorothea. Dorothea is very spiritual, and is always thinking about the state of her soul or of the poor peoples' cottages. Celia, on the other hand, is very grounded and has a healthy share of common sense.
Even her name indicates this difference from her sister: "Celia" means "sky," but, given Celia's earthiness, it seems more likely that George Eliot meant "Celia" to be an allusion to a poem by Jonathan Swift called "The Lady's Dressing Room." The poem is about a woman named Celia, and all the makeup, tweezers, and support-top panty hose that go into making her look perfect (although artificial) when she goes out at night. The poem, which was written in 1732, is meant to satirize the ways that women tried hide their earthy, physical, humanity (smelly armpits and all), to try to disguise themselves as angelic, ethereal images of feminine perfection.
Celia Brooke might not have smelly pits, but she's not afraid to talk about such things. She's down-to-earth and much more interested in discussing the proper way to wash a baby than the spiritual concerns that Dorothea finds so absorbing.