My brother is a super charismatic salesman, and I am the engineer. My parents are rather different people as well, so I'm not ruling out genetics, but I can point to differences in development. Overall I believe charisma is developed, and genetics along with environment drive your interest areas for development.
From the beginning, my brother was interested in "people" whereas I was interested in facts. I loved reading non-fiction books whereas he practiced reading people. He liked and expected to be at the center of attention, and always knew what to say. On the darker side, he knew how to push people's buttons and he was a bully.
Still, the lesson is that charisma didn't happen overnight for charismatic people. By the time you noticed it, they already had decade(s) of practice. The way for you to get charisma is to start getting genuinely interested in people so you learn about them. That's when you start to be able to push people's buttons, for good or for bad.
Interest can come from many reasons and it's up to you to discover them.
As an endnote, I was probably as socially inept as you can be. Worse than inept even as I would do or say things that piss people off. Through awareness and correction, over a decade, I can say that when I am "on" I think I can be just as charming as my brother. Of course, I'm still an introvert at heart and am not worked up to be "on" very often :)