In the eyes of the law, parents have no responsibilty for their children once they reach 18 years of age - and also children have no responsibility for their parents. They are independent legal entities with no legal obligation or interdependence on each other.
It is not permissible to assess a benefit on one independent adult based on the income of the other legally independent adult.
This area of law was trashed out in an Ombudsmans report in the mid to late 1990s looking at assessing family incomes for elderly parents in nursing homes - was the genesis of the arguments which led to the abolishment of such fees and the current repayment scheme scandal and also (though never admitted by Government, who used it for political gain) led to the abolishment of third level fees.
I dont see how maintenance grants could possibly be based on parents income - there is no legal basis for this. I suspect there is an element of "try it on" with the local authorities (?) who administer these grants and that anyone who ever complains does not get assessed. I smell another "nursing home repayment" scandal here and IMHO, any public service officials who are adminstering the scheme in the knowledge that it is illegal should be fired.