Meine Mutter is nominative in the sentence you gave, Sie ist meine Mutter.
it receives the action of the verb.
To my mind, the notion that your mother is receiving the action of being makes no sense. Even in sentences that have recipients, they're often dative so this is not a reliable test.
To suggest a better way to determine if a noun phrase is accusative or not:
An accusative object may be promoted to a subject (nominative) by passivisation. So to test if it a noun phrase is accusative or not, you can simply try to change the sentence to a passive one:
*Meine Mutter wird von ihr geworden
is a completely incorrect sentence, just like the English equivalent "my mother is being been by her". Therefore, meine Mutter in the first sentence cannot be accusative.
In contrast, a sentence like Sie sah meine Mutter would become Meine Mutter wurde von ihr gesehen. Since that sentence is good, we can deduce that meine Mutter in Sie sah meine Mutter is accusative.
Generally, copular verbs take a nominative case in German (they may take other cases in other languages, but as far as I know, never accusative). And so sentences involving copulas as the main semantic value can not be passivised.