。
She has that big house all to herself.
Literal
She [topic-は] that big house [object-を] hogging [target-に] does-progressive.
独り占め (hitorijime, 'one-person occupation') is a noun used here in the X を Y にする pattern — literally 'has it set as her sole occupation,' meaning she's keeping it all to herself. The ~にしている form combines this with the resultant-state ている, describing her ongoing situation. Carries a slightly playful or critical nuance, depending on tone — could be envy ('lucky her') or judgment ('she's hogging it'). Common in casual speech about people who refuse to share something desirable.