To deduct mortgage interest, you must be either a legal owner (on the deed) or be a beneficial owner, and you must actually pay the mortgage payment. Beneficial ownership is a bit complicated, and I am not 100% sure that only being a co-signed makes you a beneficial owner (but I won't disagree that a co-signer could be a beneficial owner, it's just that it may be more complicated than that.) More importantly, you must be the person who actually pays the interest, or at least part of it, and you can deduct the part you pay.
To deduct property taxes, you must be a legal owner of the property against whom the taxes are assessed, and be the person who pays the taxes. There is no deduction for being a beneficial owner even if you are the person who pays the taxes.
