If there is one thing I have learned about coming out it is this...
Coming out in itself never equals acceptance.
Acceptance comes from understanding. I agree with what has been said here in that trying to guess what each other is thinking is the blind leading the blind. The very first family member I came out to was my sister. She had already questioned things like why I groom my brows, why I shaved my legs, and so on. I had fear beyond fear, but was forced into the situation when I needed her to tend to my house while I was in the hospital. A house that at this point was covered with girl stuff
What was her reaction? "Mehhh no big deal, my best friends dad is fully transitioned and is now Eddie instead of Ed." Needless to say, all my fears were unfounded, you just never know. What I was good at doing, was explaining to her the "Why?" So that she understood that it wasn't just a fetish or fling. This is how I have approached everyone I have come out to and forward. I now live full time with the exception of work, and as of January 3rd, I will be out at work. This has been my biggest challenge yet and has involved not only coming out to my co-workers, but also every single one of my 40-50 clients. My approach has stayed consistent. The lesson I have learned each step of the way is that you cant just out yourself, it has to be planned and well thought out based on the individual relationships you have with each person involved.
Even with all of this, I still face my challenges. The same sister that has been supportive all the way for me told me this weekend that her and her husband agreed that me coming to there house as a woman would be too much for them to have to explain to their children and had the gall to ask me to "de-gender" myself if I was coming over. Then in further discussion about using my new name while in public, her response was to criticize my fashion, hair, and even the fact that my name wasn't "hip" enough. I was hurt beyond imagination by this as it was coming from someone who had earlier this summer told me, "You do what you need to do to make yourself happy, and I will support you 100%." Obviously we needed to have another "understanding" talk. I revealed to her things that I had not in the past discussions. Things like I was so depressed as a child that I was a cutter, the fact that on two occasions in my life I downed a bottle of Tylenol, and on one occasion held a gun to my head. I further explained that this depression was a result of trying to repress what was going on in me. This depression also led to 20 years of heavy drinking and bad life decision, all of which led to 3 heart attacks in the last 5 years and I haven't even reached 40 yet. I further explained that after my last heart attack, I checked myself out of the emergency room because the pain was gone and dying made more sense that not being able to live my life the way I am supposed to live it...as a woman. We now share an even deeper understanding on this issue. The result is compromise. I agreed for the short term to at least come to her house in a "gender neutral state," while she agreed that she needs to start preparing herself, her husband, and her children with the fact that if they want me in there life they will have to come up with a plan on how to handle this with the very young and impressionable children, who after a few years will probably forget all about the fact that their aunt used to be an uncle. The important lesson here is that I could have gone off all 1/2 cocked about all this, and believe me the temptation was there, but the outcome would have not been nearly as productive towards my eventual goal of 100% acceptance by this part of my family.
Conclusion? If you want to come out to you're sister do so, but do it with a plan and be prepared for the fact that it may not go your way initially. If you plan it correctly though, and leave it open for discussion that leads to understanding, she may just go from being creeped out that her brother has better nails than hers to an understanding that underneath everything she actually has a new sister!