So basically what I remember correctly, at the beginning the dad and brother saw the girl with red hair(who has no blood relation to the btw, hence the people in the comments saying she is a cuckoo bird I think that’s the word) they thought Soya was the fake one and the fake was the real one due to the hair color aka betraying Soya
The only one presented on the delivery room was the midwife. Her father held her after she was cleaned. So everyone thought she was switched with the midwife's child plus her feather's not red like her clan color. At first her father believes she is his daugther for he believes she will present as a phoenix, but after she presented he begin to doubt

... I'm pissed trying to understand that red haired dad...why was it again that he was convinced that red bitch is the real daughter? I mean isn't he present when Soya is born?