It's so wonderful that you are helping DS and DIL out like you are. Two adult women in a home is no easy task.
It sounds to me like maybe she feels like a guest in your home and is uncomfortable asking, or buying or just delving into your kitchen. I would just come right out and ask her. Maybe the family could sit down and divvy up some household chores.
You could ask her mother what some of her likes are, if she knows what she likes to do. Scrap booking, sports, things like that. Her favorite food.
I just wonder if she simply needs a little prodding.
Good Luck and Bless You.