Hello Amebix: Thanks so much for sharing your story. I have to admit that reading it brought more than one tear to my eyes. I'll just mention, for reference, that I'm an older man who has struggled all of his life with gender identity disorder. This is not relevant except that having struggled with this my entire life has I think (or perhaps hope) given me a certain degree of compassion for people who struggle with other issues that are, how can I phrase it, "out of the mainstream", so to speak.
One of the things I thought about while I was reading your post was a program I watched recently on our local public television station. It was about twins who were raised apart & yet how much alike they end up being. I know you & your sister are not twins. But you share a fair amount of genetic material still & it sounds like the two of you just have perhaps a lot more in common than non-twin siblings typically do. This may be somewhat unique. But considering what is known about twins, I don't know as it's all that big a stretch to imagine that two non-twin siblings could conceivably share characteristics similarly. At least that is my sorry attempt at a quasi-scientific explanation for what you & your sister are experiencing.
You know... if you continue to read through relationships posts, here on PC, over time you'll come to realize how rare (& challenging) true love relationships are.

Finding one's true "soul-mate" in this world is something special.

And, from what you wrote, it sounds as though there is at least the possibility that you & Jen are "the real deal."

Perhaps it's too soon to know that for certain. But the potential seems to be there. I personally don't see anything wrong with this. And I believe if you went back in history, you'd probably find lots of examples of siblings who loved one another & lived quietly together their entire lives... "under the radar", so to speak...
There are only two main problems with this to my way of thinking. One is the two of you, of course, can't have children of your own for genetic reasons, although there are certainly ways around this nowadays... adoption, sperm banks, etc. You also may not be able to be legally married due to restrictions written into marriage laws. (I'm not really sure about that one.)
The other problem... & the more convoluted one I suppose... is how the two of you feel about all of this. Can the two of you get past the feelings you perhaps have that somehow your relationship is "wrong" or "dirty".

I know something about living with those kinds of feelings because it's the way I felt for many years about my gender identity struggles.

But the fact is (at least I consider it to be a "fact") the feeling that somehow yours & Jen's potential relation is somehow "wrong" is just an opinion... a prejudice... based on artificial morals dreamt up by society & religion. There's no objective reality to any of it. It's simply a bunch of thoughts in peoples' minds.
Still social mores can have a powerful influence on people. There's no getting around it. So the question is, can you & Jen "unfetter" yourselves from these social mores enough to be together & not be so weighted down with guilt & shame that, over time, it corrodes your relationship. If the two of you can get past that, then perhaps you have a chance. But if, realistically, the weight of society's expectations is going to overwhelm either or both of you, then better to end this relationship now... hopefully in such a way that you can both move on feeling okay with it. And if that is to be the way it is going to be, then perhaps some counseling / therapy services of some sort or another could be helpful as a means of bringing your relationship to a successful conclusion. My personal opinion, for what it's worth, is that simply blocking Jen on Facebook is not the way to bring this to a conclusion.
I see this is your first post here on PC. So...

to PsychCentral… from the Skeezyks!

May the time you spend here be of benefit.
PsychCentral is a great place to get information as well as support for mental health issues. The more you post, & reply to other members’ posts, the more a part of the community you will become.

Plus there are social groups you can join & chat rooms where you’ll be able to connect with other PC members in real time (once your first 5 posts have been reviewed & approved.) Lots of great stuff! So please keep posting!