。
She isn't my mother — she's my eldest sister.
Literal
She [topic-は] my mother is-not, my number-one above older-sister [explanatory-なんです].
The contrastive negation pattern Xではなくて + Y identifies what something is by first denying what it isn't. 一番上 ('the very top, the eldest') uses the superlative phrase 一番 ('number one') with positional 上 ('above') — applied to siblings, it specifies the eldest. The explanatory なんです (after a noun-predicate; な + のです) frames the whole statement as a clarifying explanation, fitting a context where the speaker is correcting an assumption. The kind of line you'd expect when someone has just mistaken a sister for a mother — a substantial age gap perhaps, or a family-resemblance moment.