Newsflash: Deceased mortgage co-signer

My mother co-signed for a mortgage for my brother last fall in Nevada. She died this past March in Washington state. I am the trustee of her estate. I am struggling with the mortgage company about whether my mother’s estate is responsible for the co-signed loan. The title/deed states that if one person dies (mom or brother) the other gets sole possession of the property. I have forwarded all the trust and will documents along with the death certificate to the mortgage company. They know she is deceased however they made me a successor in interest, which according to google means that I now have some financial liability with respect to the loan. (Yikes! not what I want). However, mortgage company states that successor in interest just means I can legally talk to them about the loan. My question to the mortgage company is pretty simple, is the estate liable for the loan should my brother default? The answer to this question dictates how I distribute the assets of her estate. If the estate is liable then I keep all the assets, money, until the home is refinanced solely in my brother’s name. FWIW, none of the other beneficiaries of the estate want a piece of the LV property. At the time of my mother’s death the house was not listed as part of the trust, however, there is a sentence in the trust, “any assets not listed in the trust at my death becomes part of the trust.” So I guess that makes the LV house an asset to be dealt with through the trust? And, if you are this far, my brother does not qualify to re-finance the home on own. Any help, insight, knowledge, etc. is deeply appreciate.