So, headcanon...
Sanji's brother's don't have emotions, so there isn't really any need for them to have a free will, because they don't have to fight any emotions. His sister has emotions, but her will is definitely not strong enough to fight it.
So... concept: Sanji showed signs of Haki even as a child.
Link to my KoFi XD
Edit:
I just noticed how well this song works for Sanji????
Like: (...)
Here only the strong survive
Don't need to sign, nothing divine's gonna save me
I'm a machine, building a dream is what shaped me
Breaking the door, waging a war, no escaping
Watch me as I touch the sky
(...)