To deduct mortgage interest on schedule A, you must be an owner of the home, and be the person who actually pays the interest. You don't have to be a named borrower on the mortgage. However, since you don't pay the interest, you can't deduct it.
In some cases, if your sister paid you rent (such as, exactly equal to the amount of the mortgage), you could report the property as a rental property and deduct the interest as a rental expense. But this won't be allowed here if she is a co-owner of the property.
