FAFSA does not consider siblings' private school expenses. Period. I believe that on the Profile it
might be considered, but I've got to be honest with you - if you can afford to put 5 kids through private school it's probably not going to help you out substantially with need-based aid for that first one off to college.
Also, I don't think anyone knows the actual methodology used in the CSS Profile, so stuff like private school tuition for siblings is in a no man's land - certain schools might throw you a little tiny extra, others won't want to hear about it, but you'll never really know which schools are which. And the problem with anecdotal information is you don't know the real reason a college gives a kid some extra money. Do they just really want that kid or are they somehow calculating in that tuition expense for his or her family?
Hope that helps, but I undertand if it doesn't. LOL You might want to start shopping HARD for merit aid at this point.