Some colleges will “count” grad school students who are still dependents and members of the household…and others just wont.
At some point, you will need to provide proof of enrollment to the college for your undergrad kid.
So…you may want to contact the colleges to see what their policies are…especially if you are needing significantly more aid.
If the colleges don’t meet full need for all…this might not matter at all.
Your grad school kid will be filing their own FAFSA as an independent student…and they cannot list the sibling as another person in college on their form.