Both your mother-in-laws income and debt will be added to yours to determine what your combined DTI is. If its low enough, you'll be fine in that respect. You will still have to show the ability income wise to at least make the mortgage payment on your own. Also in keep in mind, your mother-in-law most qualify credit score and credit history wise as well. Hopefully both are good enough. Lastly, if her credit score happens to be lower than yours, that will be the score the lender will use to also determine your interest rate and closing costs.