My daughter has a half brother she most likely would not have found out about if I had not told her. I knew he lived in the area and had bad thoughts of them meeting as teenagers, etc. etc. So as soon as she was old enough to understand, I think she was around 12 or 13, I told her about her step brother. Then left her to deal with that information any way she wanted to - but being always ready to support her or answer any questions she might have.
I believe once she was older she did met him, or at least they are "friends" on facebook. (Her father did everything in his power to deny paternity but ultimately lost the case via blood tests.) I would be there for your son, but it's his half sister. Let him work out his relationship with her how ever he sees fit. It may be an unpleasant learning experience at first. But in time, who knows?
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