The assessment system does not take into account support that may already be being given to a sibling and therefore, may make university education financially unviable. Automatically allow the full maintenance loan, including the higher amount to additional siblings.
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It is unfair to expect parents who may be able to afford to support one 'child' at university, to then support two without a change in the assessment method. It is also unfair to make it financially difficult for the sibling to access higher education because another sibling got there first.