Visitor visa for my parents and niece who is adopted by my parents

[ValliG]

My parents are planning to come to USA on visitor's visa for few months. They are planning to bring my niece with them. My niece is 10 years old. They need to apply for Visitor's visa for themselves and my niece also. They are planning to go for visa appoinment in March.

To give you a little background. Recently in January 2012 they adopted my niece , in relation she is a sister to me now . She is my sister's daughter.My ister and brother are in India. Can someone let me know what would be the best approach to make sure my parents and my niece get their Visa.

My parents going by themselves for Visa appointment and then later taking my niece for her visa appointment.

All the three of them going together for Visa appointment.

What would be the chance of my niece getting her visa if she goes by herself?

What are the documents my parents should provide to make their case strong?

What are the documents they should provide for my niece to make her case strong?

[immihelp]

Why did they need to adopt her?

[ValliG]

My parents need to adopt her because my sister is a divorcee and she wanted my niece to have her maternal surname i.e my parents surname in her passport. My sister is remarried.