。
She is claiming the inheritance.
Literal
She [topic-は] that inheritance [object-を] is-demanding.
要求する is the formal verb for 'demand, claim, request as a right' — distinct from the gentler 頼む or 求める, it carries assertive, even legal force. Pairing it with 遺産 ('inheritance, estate') sets the scene squarely in legal/familial territory. The ~ている is current ongoing — she's in the active process of pressing the claim, not a one-time act. Inheritance disputes are common enough in Japan that the vocabulary around 相続 (legal succession) has its own dense ecosystem of terms; 遺産 is the everyday word, while 相続財産 is the technical legal term.