I think that the answer is more along the lines that some schools will consider the grad student sibling, but most will not (because grad student is independent for FAFSA purposes). So in planning, probably best to assume that the grad school sibling won’t be taken into account.
Keep in mind that at the graduate level, the student can borrow up to their full COA in federal direct loans – so graduate students shouldn’t have to depend on parental contributions or support - though of course it is nice when they have it.
The assumptions about FAFSA dependence/independence works both ways. When my son was a 23-year-old undergrad, living independently and paying his own way at a public college, my daughter’s private college increased her grant by the amount of son’s tuition once they received confirmation of his proof of enrollment and COA. Didn’t matter to them who was paying-- he was considered a dependent simply based on age. A year later, age 24…no such benefit.