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You can look at the link below for all the information that you need on the visitor visa:
Essentially, she must have a valid reason for visiting, must have a way to support herself without working illegally, must have significant ties to her home country, and must have the intent to return home.
A letter of invitation helps with some of this as it will help to show why she is coming to the U.S., how much time she will need and that perhaps she will have a place to stay so that she doesn't have to spend money. Usually you want to show that she is going just for a few weeks or so. If it is longer than that, immigration will wonder why and will become suspicious. They are almost sure to deny someone that says they want to visit for 5 or 6 months unless they have very strong evidences of everything else I listed above. So your letter should state why you are inviting her and that you will provide room and board while she is here visiting. But keep in mind that a letter of invitation is a small part of the process and isn't even a requirement (people mistakenly believe it is the most important) and it can sometimes be used against the applicant. For example, if the consular officer believes that the applicant will have too much support meaning that they could stay illegally in the U.S. or if there is a romantic connection, the officer would deny the visa. Sometimes it is better to not even have a letter of invitation. The most important part of a case is to have strong ties to the home country.
Now as to some of the ties, she will need to show that she has significant close family at home, property not easily sold, and a good job for a good company that she has had for awhile that she would not normally abandon.
So if she is young, and without much money or any property or a good job, she would probably be denied. They would see her as a risk for staying illegal. Also, persons that are planning to travel with their spouse and children are a risk as well because some of their significant ties are actually going with them.
I mean some people have been approved with less, but I am trying to give you the idea for the best chance.
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