These two are more like sisters than cousins, given their close camaraderie and their ability to confide in one another. Celia, sensing Rosalind's sadness, says "I pray thee, Rosalind, Sweet my coz, be merry." There is a playfulness to their relationship, with Celia beingt he instigator of the teasing, which is ironic, given Rosalind's tendency to play games and speak riddles when she is disgusied as Ganymede.
When Celia learns that Orlando is in the forest, she teases Rosalind, noting that she blushes ("Change you color?") when she mentions the necklace Orlando gave her. Celia, wanting to build her excitement at the discovery, says "Oh wonderful, wonderful, and most wonderful, and yet again wonderful, and after that out of all whooping!” ("Whooping" in this case most likely meaning "hoping") Once Rosalind figures out who Celia is asking about, Rosalind asks a number of questions about him and then says "Answer me in one word." Celia protests, saying "You must borrow me Gargantua's mouth first; 'tis a word too great for any mouth of this age's size."
Both women are separated from their lovers and help each other in requiting their affections for these men. It is clear they want to help the other towards happiness, and do so with real affection.