I jointly own my parents’ house with my sister. She prevents me from accessing the property, which she occupies alone, and refuses to pay me an occupancy indemnity. Can I compel her to do so?

Yes. Unless otherwise agreed, where a co-owner has exclusive use of jointly owned property, they owe an occupancy indemnity to the other co-owners (French Civil Code, Art. 815-9, para. 2). This indemnity corresponds to the rental value of the property and is due for the entire period of occupation. It may be determined amicably or by the court.