@blurgh123
My guess is (and I'm not entirely sure about this), Uiha may be the youngest of the siblings and simply doesn't know or at least doesn't remember a past without her brother and nobody openly talked about it in her presence.
I could be wrong though.
@blurgh123
folks are real trash at remembering the youngest years of their lives, especially the older they get.
Younger sis was portrayed as an infant when boy got adopted, so ruling out freaks like Ray from Neverland there's no way she could recall any memory that old.
@blurgh123 In addition to what the others already said:
The father made it very clear, that he considers the MC to be his son and true part of the family. Therefore there probably never was a point where they talked about the time before he joined the family (he looks to be around 5-7 at that moment, so no big need to talk about stuff before)
And why his younger sister doesn`t remember: From when are your first memories? Mine start at around 4 years old, but those are mostly only pictures and impressions, only very few "stories" where I remember stuff that was said. The sister looks to be younger than one year old when the brother was adopted, so no chance the sister could remember it.