Hi Emmy,
When we repatriated, I was the sole sponsor (the USC) for my husband (UKC).
As a full time housewife for over seven years, I had absolutely no income (zilch), and was concerned about this too.
What I did was contact the IRS at the London Embassy. I explained my situation and in turn, they sent a form letter to me which stated that, if your income is below $XXXXX, you are not required to file taxes. The I-864 does state somewhere that if you did not file taxes, to include an explanation why. I wrote such a letter then referenced and attached the form letter from the IRS. We had no problems.
Of course, this is based on our experiences and circumstances. I would advise you to contact the IRS at the Embassy directly and discuss your personal circumstances with them so that you take the right steps that pertain to you.
Good luck Emmy!!!!!