I think they have confused the two nieces, but of course I can be wrong. I wish I had access to this book!
"Eine weitere Nichte, C. Reynier in Lausanne, betraute am 23. September 1786 Merck mit der Nachlaßregelung ihrer Tante Fises, die von der Tante Katharina (»der« Ravanel) ein Etui geerbt habe; vgl. Brief 85 2. - ; vgl. von d'Epreville, 18. Nov. 1786;vgl. Brief 864."
"Another niece, C. Reynier in Lausanne, entrusted Merck on 23 September 1786 with the estate of her aunt Fises, who had inherited a case from her aunt Catherine ("the" Ravanel); cf. Letter 85 2. - ; cf. von d'Epreville, 18 November 1786; cf. Letter 864"
"Madame Reynier und der Zweibrücker Kabi-nettsekretär d'Epreville betrauen ihn mit der verwickelten Testaments-vollstreckung der Schwestern Ravanel (Briefe 852, 864)"
"Madame Reynier and the Zweibrücken cabinet secretary d'Epreville entrust him with the complicated execution of the Ravanel sisters' will (Letters 852, 864)"
C. Reynier in Lausanne was Caroline Reynier nee Chapuis whose husband died in 1773. I think they are referring to her when they say 'their husband was already dead in 1774'. I have the partial image of that page but it's too big to attach here. Anyway, you can find it on Google Books.
The page I have attached in the previous post (page 341), which is not available on Google Books, is definitely referring to the 'Eine weitere unverheiratete Nichte lebte in Holland', who is mentioned by Joseph "The attached power of attorney, drawn up in Dutch, tells me that in the place she lives, no one understands French or German", so she couldn't be the one who lost her husband, because she was single. And they couldn't possible know that her husband was already dead in 1774 because they knew nothing about her.
"Eine weitere Nichte, C. Reynier in Lausanne, betraute am 23. September 1786 Merck mit der Nachlaßregelung ihrer Tante Fises, die von der Tante Katharina (»der« Ravanel) ein Etui geerbt habe; vgl. Brief 8, 2. - Eine weitere unverheiratete Nichte lebte in Holland; vgl. von d'Epreville, 18. Nov. 1786; vgl. Brief 864."
"Another niece, C. Reynier in Lausanne, entrusted Merck with the estate of her aunt Fises on September 23, 1786, who had inherited a case from her aunt Catherine ("the" Ravanel); see Letter 8, 2. Another unmarried niece lived in Holland; see von d'Epreville, November 18, 1786; see Letter 864."
'124 Reynier, C., née Ravanel, from Lausanne, niece of Margarethe Katharina Ravanel 26, 140, 864, 894 Reynier, Doctor 24 Reynier(+ 1773), name and writings index 411."