How much older than her siblings is Reiju anyway? She seems to spend more time alone with her father in the flashbacks while the boys do their own thing, but that could just be her preference. She's the same size as them, she's assessed alongside them in this week's flashback, and her abilities seem to be appearing at about the same rate. The boys have no trouble keeping up with her. Nothing about her demeanor suggests she's too far their superior, or much more mature. But Ichihi was pretty specific about the four of them being born on the same day.
So was Reiju part of a different batch or a prototype (hence the zero) or what? Same genetic treatments, different mother even? And how far ahead of the rest was she born exactly?
I don't know how good of a clue this is seeing as how kawaii uguu chibified Oda's child designs have become (look at 12 1/2 year old Law vs how 13 year old Lucci was drawn a decade ago) but after rereading the chapter Reiju is actually shown to be a bit taller than her brothers. At least in this first panel where we're shown a back view of all the kids. And you can see it a bit too when she and Ichiji are out of the water looking at Sanji drown.
Oda's proportions are never consistent so meh.