One caveat: remember that, unlike college admissions, most prep schools are NOT need blind.
Therefore, if you do not fall into the “awesome-we-gotta-admit” applicant pile, you might be wait-listed if they allocate away the financial aid before getting to you. A prep school will wait list a qualified applicant they like if the applicant qualifies for financial aid, but the prep school has already allocated all their financial aid to higher priority applicants.
@jackb192 what you can do is explain the situation to the AOs and ask for their advice… but I’d be tempted to apply for FA using the more expensive school #'s. If you get wait listed and your brother goes to the less expensive school (or needs less $), your subsequent reduced need for FA might help get you off the wait list.