The petitioner (stepdaughter) will eventually have to submit an I-864 for each of you and will need to demonstrate sufficient income (and/or assets) to support the entire household (herself, spouse, children, any other member of the household, you and her mom). The I-864P shows the income levels. IF she needs help with that she can get con -sponsors and/or joint sponsors.
Read the I-864 instructions carefully.
The I-130 evidence is also quite important, read those instructions and gather the appropriate evidence for each of the relationships.