。
She was brushing her hair in front of the mirror.
Literal
She [topic-は] mirror [genitive-の] front [at-で] hair [to-に] brush [object-を] was-applying.
Note the で — for an active, in-progress action like brushing, the location takes で (location of action) rather than the に that would mark a resting position (as with verbs like 立つ or 座る). ブラシをかける ('to brush, apply a brush') uses ~をかける ('apply X to'), where the surface being brushed (hair) is marked with に. The same pattern appears in 鍵をかける ('apply a lock' = to lock), 電話をかける ('apply a phone' = to make a call), 迷惑をかける ('apply trouble' = to inconvenience).