question about Apply GC For married siblings

Hope someone can explain me some on this.

Is that true, other family members can be added on to the GC list if the applier's GC got approved first?

My brother-in-law's sister applied GC for him back to 2000, with only he himself on the file so far.
I though the applications should have my sister's anme and their kid, however he was told this is not necessary, because they could be added in after my brither-in-law gets his GC?

Is this true?