
Well if explain it differently, mom is a fairy. Because of people keep experimenting on her for long time she run away to future where she met dad and fell in love and had 3 boys. But because it was not her real timeline she had to go back time while she was pregnant with mc. Unfortunately when she did go back a few years before she met dad and gave birth, she has gone mad and dead.
If mom hadn't went back in time and gave birth normally mc would've been 5 years old which could explain why the potion turned her 5 instead of the age she wanted and also could be why it's so hard to turn back to adult.
But last part is just my theory

About the last part, I read somewhere that she turns 15 after she got the crown which I'm a little confused by cause she IS the youngest. She did get born a little bit in the past so her age makes sense but i was hoping that she'd stay little cause her big brothers never experienced having her as a little sister long enough. I feel like the growing up part is forced so it wouldn't be too weird to pair her up with the ml

Yeah it is complicated due to the mixed feelings of wanting to be with ML but also wanting to catch up on growing with her family. I guess she kinda took a “middle” decision. Spoilers say she wanted to stay with her family and chose to grow up with them (but she also liked the ML so she promised to be with him later). She doesn’t get with ML until the side stories (though they confess before that)
This is getting more confusing