Is it permissible to cease enjoining good and forbidding evil to a half-sister, if that causes insult, verbal abuse, her father to describe me as extremist, and for him to undertake her upbringing?

Enjoining good and forbidding evil is legislated, accompanied by patience in enduring any harm that may befall a person. If the one advising fears harm or if their denunciation leads to a greater corruption, then they have latitude to forgo the denunciation. If advice to the sister is left unspoken, the person does not become a dayyūth. Her guardian is her father, and this should be explained to him gently and respectfully, without offense or excessive elaboration.