It was my understanding that as a US citizen, you could petition your parent and this would automatically include any children under 21. My sisters are 19 and 16 years old. I just received a processing fee bill which only includes my mother and not my 2 sisters. I called the National Visa Center and they said that they could not guarantee that my sisters could come with her. She is divorced and holds custody for both of them. What should I do now? Why aren't they included in the petition? What is the standard procedure in a situation like this?

Some petitions allow a person to “add on” or include certain immediate family members whether those family members were named in the original petition. These family members (such as spouses or children), called “derivative beneficiaries”, may not have even been around at the time the petition was filed.

Anyway, this does not apply when you file a petition for your mother or father. You must file separate petition for each. It may take up to 24 months for your mother and 12-25 years for your sisters.