What are the laws on threatening someone?

long story short, my brother is in jail because his wife bribed her 15 year old neice into accusing him of molesting her. now my bros wife is dating another guy and sent him a letter threatening him, telling him that he had better "tread lightly cuz im on my last nerve. you better not piss me off again." and so on. the whole letter was really menacing and mean spirited as well as she slanders him and trys to desimate his character and intimidate him. what can i do?