Being on a mortgage or paying the mortgage for someone does NOT give you title to the house. People can and do pay for others (e.g. family members) without becoming owners: it would take a written agreement that in exchange for paying, the payor will get title in order for him or her to get it (agreements affecting ownership of real estate must be in writing). Furthermore, the payor cannot even sue for the money paid unless there was an agreement to repay which was violated: voluntarily paying on behalf of another ("gifting" money or payments to them) does not obligate the other to repay in the absence of an agreement. So any rights the payor has will depend upon what had been agreed to before they made the payments.