In our experience, it seemed that some CSS colleges did factor in younger siblings’ private school tuitions when calculating aid. It is hard to know for sure, but the question was asked on the CSS for some colleges, and I think that the tuition I pay for my D22’s siblings might have been part of the aid differences that we found at meets need schools. However, I doubt that private school tuition would be a factor considered for a family with a 400K income, particularly since most such families have substantial savings and assets like home ownership.
Another way of putting it, if the OP’s parents don’t qualify for aid at their sibling’s private high schools, their choice to send the kids to private school seems unlikely to make a difference in how the colleges calculate need. Instead they might look at the money going to the siblings’ schools as discretionary income that could be spent on college tuition instead. But that is just my guess.