I fear I'm going to come across as a jerk writing this but I think it is necessary. I respect Epiphany's reply, but I'm less of a determinalist than he seems to be, so I'm afraid I personally don't believe you had no choice (perhaps, limited choice, or choice informed by desires, but choice none-the-same).
First off all, I do not think you are a monster, you are a human being (as are we all) and as such I think you have some intrinsic worth. I can also see that you really want to believe that you were in some kind of mutual relationship with your half-sister. However, you need to fact facts (and I'm sorry, but I really do think that they're
facts).
A 10-year-old infant cannot consent. A 14-year-old child might be able to consent with someone her own age, but not with someone almost decades her senior. Sorry.
I think you've got to get out of seeing this as a "relationship" or "sex" and realise that you absolutely committed rape and child abuse.
Now:
please do not take this as meaning I think you should kill yourself. I don't. However, I think you do have to face facts that you did something gravely, gravely wrong. Please seek therapy about this immediately. I believe your sincerity when you say you would not repeat this behaviour, but I think you need to work through this, for others' sake as well as your own.
As for turning yourself in - personally I think it depends upon what would be best for your half-sister. Personally, I think the best thing to do would be to send her a letter precisely once, stating plaining that you sexually abused her and owning this crime. You should then ask what she wants you to do, including any legal reparations. Do not try to justify or ask for forgiveness or explain away. Also, promise that if she does not want to respond or do anything with the letter, that you will
never contact her again.
Download this and read it and please take it to heart and follow what it says:
http://www.phillyspissed.net/sites/default/files/what%20to%20do%20when%20someone%20tells%20you.pdfBest wishes,
Sprock
-- Thu Feb 27, 2014 6:49 pm --
EDIT: This pdf is tailored more towards people in otherwise consensual relationships who have pushed someone's boundaries or otherwise assaulted their partner. I have detailed the fact that I feel I sexually assaulted my 16-year-old partner, when younger, on this forum (i.e. this is my definition, she has said her 'boundaries were punished' and that she felt 'really uncomfortable'), so I do not want to elevate myself into a position of judging you. however I think the second page with the sections entitled 'Take Responsibility for Your Actions' and 'Seek Help' are applicable to you and you should read them carefully.
-- Thu Feb 27, 2014 7:09 pm --
EDIT no.2: Upon re-reading some of the advice really is only relevant to those in relationships.
OK. In short - not legally binding advice:
1.) Get therapy now. Sooner rather than later. If they do not currently seem you to be a risk to children then you will not be reported. I will you have to leave that to their discretion, even if you are scared.
2.) If you think it's the right thing to do, send a letter to your half-sister, stating simply that what you did was wrong and illegal and unequivocally your fault and that you are happy to take any legal consequences she might wish. Also tell her that if she doesn't want to write back to you, she will never contact her again.