Responses
I think its general knowledge he always knew since she is nobility
No. Just double-checked. In chapter 56, she used 'we', 'us' and 'our' when talking about what her father did for them, which caught Ignis' attention in chapter 57. He was then curious if Celestine had siblings and wanted to go back and check. I just don't remember if he ever did.
??? Ignis said "father, sister, and fiance". When did he find out that Celestine had a sister???