Yep, that's definitely a "she". They all develop at their own pace...the more developed one will probably start laying very very soon, and her sister will follow shortly. They both are beautiful little pullets. For comparison, here is my RIR roo at only 15 weeks old...notice his huge comb and wattles (even bigger now at 30 weeks), and also his glossy hackle and saddle feathers:
Thanks...I think he's pretty awesome myself. I need to get new pictures of him because he is even more impressive now, especially his beautiful tail feathers.
If I were you, I would be checking the coop for eggs every day!