First my condolences. Second, the past due obligations are those of your mother's estate, not of her heirs at law (you and your sister)personally.
If there are credit card bills, check to see if perhaps they were covered by life insurance built into the credit card agreement as a benefit or as an extra option for a fee. (Be persistent -- the leading credit card insurer usually denies the benefits at first even if there is coverage. Lots of people have "credit life" but most don't collect on it as they forget it or take the insurer's [false] word.)
Third, if there are any assets your mother had, jewelery, a car, furniture, etc. then they are all part of your mother's estate, and their value or other proceeds of the estate should be used to pay her estate's bills. In other words, you can't legally take the car, the jewels, etc. and say to her creditors "sorry". But if she had absolutely nothing of value, and there are no assets, you don't have to pay them personally as a matter of law. PS Usually the life insurance money is yours, not the estate's, if you were the named beneficiaries.
