。
She lost one of her close relatives.
Literal
She [topic-は] close-relatives' one-counter-of-people [object-を] lost.
肉親 ('close blood relatives') is a slightly formal compound — literally 'flesh-relative,' i.e. people related by blood. 失う ('lose') used with people typically means losing them to death or estrangement, distinct from 失くす ('misplace [an object]'). 一人 here is the counter-form for one person.