12 weeks is young for sex feathers on a Brahma. I'm leaning toward male for the dark due to the thick legs. Probably the best way to sex them at this age is a good comb shot. Pea combs: single row is female, triple row is male. I can't see them well enough in the shots to tell, you can tell on the live birds.
Your Light Brahma could be a pullet, my baby chicks are 10 weeks old and their faces have started turning pinkish red colored (though they are not the same breed).
I'm pretty sure that your Dark Brahma is a cockerel due to the thick legs, tall stance, and more developed comb and wattles. I'm not sure on the Light Brahma, but the stance makes me think cockerel.
the color pattern on the dark brahma is what is making me say cockerel. you could post on the brahma thread and they could probably tell, since they specialize in that breed.