。
She lives directly across from us.
Literal
She [topic-は] our directly-opposite [in-に] lives.
真向かい (まむかい) is 'directly across, right opposite' — the 真 ('true, exactly') intensifier turns 向かい ('opposite side, the side facing one') into a more precise 'right across the street' or 'directly opposite.' 住んでいる is the standard way to say 'live (in/at)' — bare 住む would be the punctual act of taking up residence; ~ている is the steady state of residing. The に here marks location, where she lives.