I'm trying to help Mom with her taxes. Last year, she moved into a new home with my sister and brother-in-law. All three are listed as Payers/Borrowers on the 1098 Mortgage Interest Statement from the bank, but, there is only one Social Security Number listed (BIL's) and thus is reported to the IRS against only this number (note at the bottom of form 1098 states 'Interest will be reported on the Social Security Number for the Primary Borrower above'). Mom pays 25% of the mortgage and thus entitled to deduct 25% of the interest paid. Question is: How is this handled on tax form so as not to set off bells & whistles at the IRS (mortgage deduction claimed, but, not reported to IRS with Mom's SSN)? Any advice would be appreciated!