No. She is the daughter of Albert's nanny. Elijah's mother and Albert's mother were both pregnant at the same time, however Albert's mother had a son a second son who died shortly(?) after being born. Since then Albert's mother has had health/mental health problems and so when she is doing unwell she thinks that Elijah is her dead son Phillip. To help keep Albert's mother from getting sicker they've had Elijah raised as a boy so that the Countess can think she is her son.
I thought that albert’s dad had like an affair maybe I read it wrong...
I thought so too, i thought albert wast the one who came out of the affair and avery/eijah was the one born into the marriage but the mom was tramatized by her sons death and mixed up her son and daughter...
http://www.mangago.zone/read-manga/my_fair_footman/hs/mpn_chapter-2012636/pg-1/
This is the chapter that speaks about this part of the backstory, I had a bit of a hard time reading it as for some reason on my phone its blurry. However, as far as I could tell Albert was not a product of an affair, and his mother gave birth to his brother Phillip who died shortly after being born. Elijah/Avery?, was born from Albert's nanny (a maid?) out of wedlock. Because they were born so close together and Albert's mother couldn't handle the death of her second son Phillip to help her stabilize they had Elijah live as a boy and in the instances when Albert's mother wasnt doing well they would let her believe that Elijah was Phillip.
wait isn’t Elijah technically the male leads half sister? I’m so confused