The financial aid application for boarding school that I filled out specifically asked about educational costs for siblings. So yeah, not an unreasonable question.
How prep schools award financial aid is a little different from the college process.
Because most prep school students are minors, there is little to no expectation that they pay for their education (and kids can’t take out loans) - the parents’ ability to pay is all that is taken into account. (There may be some exceptions, but aren’t there always?)