Hi Amebix,
I do understand I had feelings for my cousin he is a first cousin and he didn't have a clue or feel the same way,I know sister and brother is way different,some cultures condone cousins together but none sisters and brothers or half sister,half brother in your case.

I agree with Skeezyks you can't just block her on Facebook and end it without explaining to Jen.It's not air to her leaving her thinking the feelings are just on her part and like you say that you are disgusted.You have to at least tell her you feel the same but don't want to go there.

If you discuss this there will probably be an either or conclusion.You either both agree you don't want to go there and agree to limit your connection and contact each other less and find other partners,or you will ignore the societal norms and explore what is going on and take this into relationship mode without the guilt and shame,or with guilt and shame.

With guilt and shame you could end up destroying each,guilt and shame leads to self hatred and eventually you will hate her and blame her for that.And you will hate yourself and be consumed with all of the negativity of it all.Other possibilities you get the sex together out of your systems and it turns out to be just lust which burns out and you can then go back to being brother and sister,kind of a familiarity breeds contempt scenario but maybe in your case you can become friends and go back to being just half brother and sister.

I am afraid if you just block her and refuse to see her you will build a tension around this that will be unbearable for life and unresolved for the both of you .

So you have to follow this path and see where it takes you and her.I am sorry it is so hard and I have loved people that were a taboo in my culture,I took the coward's way out that you are thinking of taking,that way lies an equal amount of pain and regret as if you just go ahead and allow the feelings and intimacy,at least allowing it might help it to dissipate and fade to zero.

It is your decision I hoped I have helped a bit to make you come to a decision and see what the options are.
I wish you both well.Whatever the decision I think the guilt and shame are unnecessary,you can't help the way you feel and your body reacts,it is true that the genetic factor makes it much more intense,but don't fight what you can't control unless of course it is your choice 100% not to go there and you feel you can control that without either of you getting hurt.I wish you all the best with it.