> Moreover in such a rich family in France, changing identity like this would have been spotted really quickly.
She sold her apartment for a fixed per month amount. Sure, you need to have the apartment in the first place but that doesn't sound rich to me.
Further, if my neighbor dies, I do not have any insights in the paperwork. So at that point, I only know that that nice older lady from upstairs died. If you then ask me 60 years later if that older lady was the mother or the daughter, I am not sure I would know. Besides, I would probably be proud of someone like her in my neighborhood, so I might be careful to refute her. Afterall, you get a lot of prestige from having the longest lived person in your country, city or neighborhood.