‘Your parents HOLD in your siblings’ names’ are accounts parents own. If the account is held by the sibling, it is not a parental asset.
Schools can ask whatever they like. Some are okay with not asking about 529 plans for siblings, other schools want to know because they know that money is actually available for this student. A parent could put all the money into the 529 for the youngest child and just move it every year for the older kids. Schools know that. Some care, some don’t.