、。
She's not my older sister — she's my mother.
Literal
She [topic-は] my older-sister [is-not-ではなく], mother [explanatory-なんです].
A textbook contrastive correction using ~ではなく (the negative te-form of だ): 'not X, but Y.' The clause-final なんです (the explanatory ~のです) frames the second clause as 'the explanation' — 'the situation is, she's my mother' — softening what could otherwise sound abrupt and casting the correction as informative rather than confrontational. Practical context: this kind of clarification often happens because Japanese mothers in their thirties or forties can look strikingly young, especially when out together with their children, and people regularly mistake mother-daughter pairs for sisters. The compliment is so common it's almost a cliché.