@Merliah,
Honestly, I think the term is made up. If you look at psychology websites, you see the traditional breakd0wn of personality types but if you search for sigma females, you see a bunch of pop culture sites, the search for the super female who blows the alphas away! More confident than an alpha! More emphatic! All of the things you love about an alpha but without any of the bitchiness!! It all sounds like bunk to me, another offhand attack on women with strong personalities. All that said, if you were a magical "sigma" female, you would not be on the Internet asking about it, you'd be living it hard.