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Giving them one dollar is NOT providing more than half of their financial support.
You would need to get their financial records to figure this out. You need to add their rent or mortgage expenses, home maintenance, water, electricity, phone and other utilities. Then you need to add in their clothing, their food, medical care and other expenses. Take half of that value and you will have 50% of their support costs.
You must be legally responsible for repayment of the loan in order to take the deduction for mortgage interest.
It doesn't matter who pays it. Your parents are the only ones who can claim it.