Undergrad schools vary in how they deal with grad school siblings in the "count" of students in college. Some allow grad school students to be counted, and others do not.
It doesn't matter if your grad school student is independent for financial aid purposes. ALL students who have a bachelors degree are independent for financial aid purposes...they do not have to be 24 years old.
On the FAFSA, you can count the grad school student as a member of your household IF you provide them with more than 1/2 of their support. You can also then include them in the college student count as pursuing a grad degree. BUT the school your undergrad student is attending might NOT allow you to count the grad school student.
When DD was a college freshman, her big brother was college senior. We asked EACH school she was interested in what their policy was. Some would count him as a grad school student...and others would NOT. YMMV depending on the school.