Letters to the Editor
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You have every right to look for and establish contact with your 1/2 sib
What you don't have a right to do is to demand that your father or mother establish a relationship. Your mother is not related to your half-sib. Your father made his own choices, whether or not you agree with them.
I will note that if you succeed in contacting your half-sib, he may be disappointed to learn that his father (the one relative he probably knows about) is not interested in knowing him.

