The ratio guidelines for this type of underwriting is (with no exceptions) 31/43.
Your total housing ratio may not exceed 31% of your combined income. Your total debts (housing plus nonoccupant coborrower rent and debts) cannot exceed 43%.
Husband's unemployment will not be counted as income.
The specific answer to your question will be from your lender. They should be able to explain whether they can use her income without her having a credit score or whether they need to have her establish a credit score. They ought to be able to do that.
But one other idea that might be worth considering. if your husband is unemployed they might be able to use his unemployment income for part of the qualification? We use it in calculations for loan modifications, i would think that maybe they'd use it for loan origination.
I'd love it if you contacted me back as to how it all turns out. We always learn from the experiences of others and that will be helpful to many other people here.
Thanks AdamK135 and good luck!
However, just because it may be able to work does not mean that you should do it. You do need to carefully consider what will happen if you husband does not go back to work, and refinancing to remove your sister may not be as easy as it sounds. Best of luck.