Hi, I am not sure if this is the same Yznaga family but... There is a Cuban Yznaga family and there is a Fernando born in Louisiana circa 1850 who then lived in New York. He had a sister named Emilia/Emilie/Emily (all three spellings show up) who spent many years in Paris. She was born circa 1860s. Fernando was married twice in NY but had no children. He died in 1901 and left his estate to sisters.
There seems to be very close similarity in these names/dates and locations. Emilie Yznaga (sister) never married and died in 1944 in NY. I think she was still in Paris as late as 1938 but probably left due to WWII. If you look up Fernando Yznaga in the New York Times you can find lots of articles including obituary. If it is the same one. Good luck!