In 1880 I believe there was a Felina Stay living with the "Forgas" family in New York.I have to wonder where these children were.However, by 1900 Felina would be about 21 so she could easily have her own family.
I also have a "Francesco Stay" born in 1882 and dead December 6, 1882. It either suggests Francesco and Franklin are twins or the same person since I assume you need approximately 9 months between babies.
The current theory is Nancy had two children living in 1900, Irving Bruett and Felina (Stay).
As a side note Franklin Stay may have been born in 1853 and there is one in Fort Ethan Allen,? Chittenden,? Vermont by 1900 Military Census.They must have divorced since he is listed as single.