回複：Your lawyer is right.

In fact, another lawyer told me that I should let NVC know about my sister's family members. On I-864, part 3, the instruction is

Accompanying Family Members You are Sponsoring.The immigrant you are sponsoring may be bringing a spouseand/or children to the United States. If the spouse and/orchildren will be traveling with the immigrant, or within sixmonths of the immigrant's entry to the United States and youare sponsoring them, you should list the names and otherrequested information on the lines provided. If anydependents are not immigrating, will be immigrating morethan 6 months after the sponsored alien arrives in the UnitedStates, or you are not sponsoring them, do not list their nameshere. A separate Form I-864 will be required for them whenthey apply for their immigrant visas.

I think I can fill this part for my sister's hu*****and and her daughter.

Thanks.