First, I would like to express my condolences on the recent passing of your sister.
To add a Dependent, once you are in your return click "My Info" on the left side of the screen and then click "Add a Dependent".
If the children lived with you over half of the year, you should qualify to use the Head of Household Filing Status. TurboTax will walk you through this when you add them as Dependents on your Tax Return. You can revisit your Filing Status in the "My Info" section by clicking "Edit" next to your Filing Status.
To use the HOH Filing Status, you must have provided over half the cost to run your home for you and a Qualifying Child. Please check out this TurboTax FAQ for a complete description of the requirements you must meet for the HOH filing status: Do I Qualify for HOH?
Based on your income, there may be other credits that you qualify for since you are claiming the children. TurboTax will walk you through all of this step by step.
