Have folk seen this website? I did a search to check and couldn't find it on here. It contains news of an interesting Larry Todd book on Fanny Mendelssohn and some recordings of her music.
https://global.oup.com/us/companion.websites/9780195180800/audio/
By the other Mendelssohn I thought you might mean Arnold, or Ludwig, or ...
It is true that Larry Todd is not short of Mendelssohns to write about.
True. And besides her considerable talent, a book about Fanny Mendelssohn offers opportunities for context. If I understand correctly, one of her grandchildren was Kurt Hensel, a name that will only mean something to mathematicians and ex-mathematicians (and would-be mathematicians) (like me) - he discovered/invented/whatever what are called "p-adic numbers" (p prime), among other things; not one of the very biggest names in mathematics but definitely a -name- (ie one that is known and whose discoveries/whatever are even more known- briefly, someone who, like quite a few people in his family tree, made a serious contribution to their fields (not just the musicians but I'm looking at you, Moses Mendelssohn :) )
Hensel - a name I recall despite having no interest in mathematics but lived at University with a household full of mathematicians.
The recordings of Fanny's music on this Web page is rather good. Especially the quartet which I hadn't heard before. I may have to look for a recording to purchase.