Younger??... Somehow I had the idea , her sister was OLDER..?
Yes, I thought so too, it has been mentioned in several books that her older sister died, leaving her the sole heir to the fortune.
Also, the parents were older, born in 1827 and 1828, it is unusual they would not have had a child before 1861. I think of this because I was born in 1961 and my parents in 1927 and 1928, and I was the third of 4 kids. Especially back then with the desire for heirs, and lack of birth control, it would be unusual they would not have a child until they were over 30, especially a woman. I have never heard of any other babies dying in infancy before Zenaida, have you?