。
She was aware of their hostility.
Literal
She [topic-は] their [genitive-の] hostility [to-に] had-noticed.
気付く ('to notice, become aware') takes the noticed thing with に, not を — a common learner stumbling block. The ~ていた form ('had noticed' / 'was aware of') marks the resulting state of the noticing: she had taken notice and continued in awareness. 敵意 ('hostility') is built from 敵 ('enemy') + 意 ('intent') — literally 'enemy-intent.'