Want to know how to write letter of consent

Hi everybody,

I will be very glad if someone can help me write letter of consent on behalf of a husband who is going to visit his brother at USA. I want the letter to states how long he or she is going to stay with his brother or sister. the purpose of visiting, i also want the letter to be directed to the consular of the USA embassy. Hope to hear something new from you all. thanks for your time and your effort.