。
She treats them harshly.
Literal
She [topic-は] them [to-に] harshly treats.
つらく当たる ('to treat harshly') is a set expression that doesn't decompose cleanly. つらく is the adverbial of つらい ('painful, hard, harsh'), and 当たる ('hit, strike') has a specialized sense of 'to deal with someone in a particular manner.' The に here marks the target of the harsh treatment. Without the idiom, the structure would be opaque — 当たる alone wouldn't suggest 'treat,' and つらく alone wouldn't suggest 'harshly toward.'