If my future wife were to lose full custody of her daughter to the father and she is a stay at home mother to our other children would I be responsible to pay child support when we get married?

Most states have a min child support due if a person is not working, my state sets it at a figure of min wage at 30 hours a week assumed income ( what they figure they could work if they would)

So you are not required to pay it for her, but I would assume you would to keep her out of jail.

Its possible depending on where you live. Also the courts can attach an amount to her and mess up her life forever if it isn't paid. Yours would be messed up too as choices you make would be limited by her ability to do things.