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If I were in your situation I think I'd all APS, explain the situation, and ask for a welfare check on Mother. If Mother claims all is well and that there is no problem then there may not be anything they can do. But it would put me on record as being concerned and not approving of the situation.
You say your mom has general age-related decline. You do not mention dementia or mental health issues. If she is willing to accept abusive behavior from some of her children there may not be much you can do about it. But that doesn't mean you have to accept abuse from her. "Mother, call me another time when you are feeling more charitable. This conversation is over now." And hang up.