@Jack of Hearts,
I strongly disagree. The OP has every right to know where he or she is from.
The father has legal and moral responsibilities to meet the needs (including emotional needs) of his offspring. The child has no responsibility to worry about the emotional needs of the father.
That being said, I think that you should be very sensitive to the half-sister. It isn't her fault or responsibility and I think you should do everything you can to protect her right to not be involved.
I wish you the best of luck in this difficult situation. I do hope that you have a good relationship with the man who raised you.