There's been threads on this before. Ornata's the most obvious Poec to sex immatures by colors/markings. With a few others, like vittata, male's colors will start to fade prior to maturity, but to a lesser degree. Understand that one shed can make a big difference, and many people have screwed up by relying on color/markings alone to sex juvenile Poecs.
Exactly. They really need to be at the same instar so you can do a side-by-side comparison. If a female's one molt behind, she might look like a male, or vice versa. With siblings it's much easier. When you start mixing different ages and sizes to sex, you can easily make wrong assumptions.
how big are the both of them when this was taken?Thought I would post a comparison between immature female and male Poecilotheria ornata.
You can clearly see the differences on the carapace markings (Lack of markings on the male) and the clear white outline on the females folio on the abdomen.
This thread is quite old (posted 2014), but you should be able to reliably tell male from female dorsally as small as 3-3.5" diagonal leg span.
Post molt picture around 3-4 days after. 3.5" in DLS now