US citizen petitioning mother with two daughters under 21
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?