Without knowing the circumstances at the time of the divorce it is hard to tell why the father isn't paying support. It ius quite possible the the fathers parents did not want child support from their son. It is also possible that the father was originally awarded custody and then lost custody to his parents without a corresponding order to pay support. It may simply be the fact that the fathers side of the family is the custodial guardian.
Whether or not the father is required to pay support will not impact what your sister pays. As you have alluded to, her current circumstances warrant a reduction in child support. To get this done she will need to file for a modification of the support indicating her current reduction in earning capacity and health as the reason. The guidelines indicate that she should be paying 14% of her income.
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Please be aware that my answer is not legal advice, it is merely information and no attorney client relationship has been formed. You should always contact a local attorney for legal advice.
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