Can My Brother Be Held Responsible For Paying My Deceased Father's Mortgage?

My father took out a mortgage on his home, approximately 12 years ago, to lend money to me and my brother. Nothing was ever put in writing. My share of the loan was repaid. My father recently passed away and my brother, who is currently unemployed, is unable to pay the remainder of his share of the loan to the estate. The money to pay the mortgage is currently being taken out the estate each month, cutting into the shares of the benefactors. My brother never co-signed the mortgage or signed a promissory note of any kind. All I have are some letters sent to my father asking to borrow money. My brother also made a check, in the amount of the mortgage payment, each month to my father, if that counts for anything. In the eyes of the court, does my brother have any legal responsibility to repay this debt which is entirely in my father's name? Can the estate sue over this? What are the chances of winning given what little evidence there is? Your response is very much appreciated.