Hi Lisa,
It takes courage to be transgender. My therapist and my wife have both told me that they see how much courage it's taken me to get as far as I have, although I don't see it as that much. To me, you're remarkably courageous coming out to your sister. She deserves kudos too for remaining calm and just listening. I'm sure she has a lot to process.
I think it's tough to be a woman or a man and it's probably impossible to really determine the relative weight of either. I recently read that although people who have fully transitioned to their natural genders feel much more comfortable in their skin they still carry a secret about their surgery: whom to tell, when, etc. We all need to be courageous.
Your post brings up something I've been thinking about. I often read how women (particularly teenagers and 20-30 year olds) develop their confidence in themselves. Okay, but I don't read that about men, whom I think are also tasked and challenged with that. I sure was (and am). Is it so much different? Perhaps as you wrote, a particularly feminine challenge is to appear "good" in any setting, with the right level of casual/formal attire, the total look including accessories, and the fit. I certainly agree it's easier for guys to accomplish this.
Last, I also agree with your consideration of over-thinking things. I think we all do this especially when we're faced with such profound decisions and steps. (I wish I'd thought more before I proposed and got married the first time!) It's hard to trust our instincts. Consider how long your gut feelings have been there despite all the attempts to suppress/ignore them.
Be well,
Emma