We are a family of 4 siblings. The eldest is single without children and wishes to bequeath all her property to her nephews and nieces. Does she have the right to do so?

Yes. A person can freely dispose of all their assets by will, as long as they have no compulsory heirs. The beneficiaries are the children (article 913 of the Civil Code) or their descendants in the event of predecease (article 913-1 of the Civil Code), and in the absence of a child, the surviving spouse (article 914-1 of the Civil Code).

Brothers and sisters are not compulsory heirs. Your sister, having neither child nor spouse, can bequeath all her assets to the benefit of the people of her choice, in this case her nephews and nieces.