My friend lives with her mom and sister. The mortgage and 1098 is under the mom's name (sisters names might be on deed/title) but the sisters pay all the house expenses(mortgage, insurance, utilities, car insurance, ect). Sister 1 pays 75% of expenses and sister 2 pays 25%. Monthly expense total is transferred into the mom's account and paid from there. For the past few years they have been taking the mortgage interest deduction on the mom's tax return (the interest exceeds her income). Can they amend all their taxes for the last few years and give the deduction to the sisters? How can this be broken down (ie. sister 1 makes alot more, can she take the full amount or do they have to split it based on their payment proportion?)

Thanks in advance