F4 Question-OK for only derivative to apply for visa but not applicant?

[AssumeZero]

My F4 petition for my sister finally became current this month. She has a son (my nephew) who is under 21 as a derivative beneficiary (shows as ACCOMPANY, vs APPLICANT for my sister) on the NVC website.

Question: Is it OK for just my nephew (derivative, accompany) to proceed with the visa application without his mother? Any issues doing that?

 

[newacct]

No. Derivatives can only immigrate at the same time as or after the principal.