From your information, your mother did have some wages in 2009. You can file a return for your parents and report that income. Your parents will claim the 17-year old child and may get some Earned Income Credit. It will be sent to you as the guardian. Although you are not personally responsible for your parents' debts, if they receive a refund on their tax return, it should be used to pay off their debts as it is their money.
Even if there is not sufficient income to require a tax return to be filed, it is a good idea to file it and close the year. If the year is left open, the IRS could find tax owed in a future year which may be a problem for you.
Contact the SSN regarding survivor benefits for your younger sister. You personally will not receive survivor benefits as a guardian.
As your sister's guardian, you will have the same tax relationship to your sister as a parent in 2010.