My older D stayed instate and we paid $18,000 a year. Younger D was a freshman at a fafsa/profile school that met full need when her sibling was a senior.
The cost of our older D’s school had no impact on younger D’s FA. Younger D’s fafsa EFC was higher than the cost of her siblings college.
I think it depends on the school. I would call the FA office and maybe use the NPC.
Btw…we used the NPC for the school that your son currently attends…with older D in college ($18,000 a year). The NPC was accurate and the school gave younger D the same size grant as the school she ultimately chose. The lower cost of her sibling’s school had no impact…at least a few years ago (at your son’s current school). Nothing was added back in. Hope this makes sense!