。
She lives in the apartment above us.
Literal
[topic-は] us [genitive-の] above [genitive-の] apartment [in-に] is-living.
われわれ is a slightly formal/literary first-person plural pronoun — more weighty than the everyday 私たち, often used in writing or in speeches where the speaker wants to invoke a collective. In casual conversation 私たち or うちら would be more common. The genitive chain われわれの上のアパート ('apartment of above of us') stacks two の links: 'us-of-above-of-apartment.' Japanese builds spatial relations with consecutive の-linked nouns where English uses prepositions. The polite ~ています keeps the register polite throughout.