Spoilers:
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She is their full blooded younger sister. The countess/mother is the fairy queen. She time traveled to the future to escape her captors and met the count. Then they fell in love and had 3 sons. Then they had the FL. The mother was pregnant with the FL and something happened (has to do with her mind going crazy and health deteriorating) that made her go back in time and give birth there. So it kinda messed up the timeline and made the FL older than she should have been.                                            
                                                To go in a bit more detail, if I remember correctly, the reason why she’s been stuck as a child is because she unconsciously wants to be with them as a family. Then later once she receives the fairy crown, she ages up to 15. She decides she wants to grow along with her family and wants the ML to wait until she’s older to get together. The ML agrees and doesn’t see her until that time. It’s actually the FL who gets impatient and is the first to reach out to him after the agreed upon time. 
Before that decision to stay with her family, I think she turned back into an adult a few times and they had a bit of romance moments.                                            

		
My theory is that the fl is those boys older half sister.
The mom lost her memories so she probably forgot about her and had a new family. If you know what actually happens pls spoil it for me