This seems like your best option, at least for the short term, until you can figure out a more permanent solution. Explain to "Mom" and her sisters that, if "Junior" gets mistakenly listed as deceased by someone or something important - like a government agency or a banking/credit company. Most likely, it would totally ruin his credit, and possibly everything she is relying on to support her in her later years ...
Can you get the phone transferred to your name? Or just cancel hers and start over?