Curley's wife can flirt somewhat openly (as long as Curley is not around) with the other ranchers. Since she is Curley's wife, who is the son of the boss, she can pretty well do what she wants. If any of the ranchers are unhappy with her behavior, she only has to tell Curley and he will have his father fire them. Curley's wife flirts/interacts with the other workers partly out of loneliness but also because she knows she can do so with no consequences. Being the wife of the son of the boss, Curley's wife feels free to do whatever she pleases. This is most apparent in Chapter 4 when she warns Crooks that she can have him hanged if he challenges her in any way: "I could get you strung up on a tree so easy it ain't even funny." Candy replies that he would take Crooks' side. Curley's wife says nobody would listen to him (or Crooks or Lennie). Candy relents. After Curley's wife leaves, Crooks admits that if it was to be his word against Curley's wife, she would win every time: 

"You guys comin' in an' settin' made me forget. What she says is true." 

Curley's wife has even more power over Crooks because he was separated from the other men, essentially treated as a second class person/worker. In general, Curley's wife has the power to toy with the other workers as long as Curley is not around. She has only to tell Curley or his father to have a worker fired.