I-864 ensures that she will not become a public charge and will not use means tested public benefits. you are not responsible for her student loan, unless you co-sign and she stops paying on it.
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You are responsible to ensure that she is at 125% of the poverty line. Generally a student loan is not a needs based public benefit, so you should not be liable for that, or other debts your cousin incurs.
The above is intended only as general information, and does not constitute legal advice. You must speak with an attorney to discuss your individual case.