Is the Home Ec curriculum available online?
I noticed you mentioned cooking. Nutrition is an important subject to know about. You didn't mention her age so it is hard to recommend what to start her with. I would not start with cookies, which are nutritionally very poor and very high in sugar. Maybe her GGM has a recipe for a family dish she wants to pass on?
When I did Home Ec we did first aid as well.
Budgeting is another important area (putting the Ec back into Home Ec)- you could work some budgeting into the sewing lesson very easily.
Good luck with it. It is a big responsibility to take on.