I can't say that to him, he's my dad. / I can't say that to her, she's my mom.
I am too close with him/her to tell him/her that.
I can't tell him/her that, he/she is my friend!
「親にだからこそ言えない」は英語ではこのようです。
父親の場合：
I can't say that to him, he's my dad.
母親の場合：
I can't say that to her, she's my mom.
"I can't say that to him/her" = 彼／彼女に言えない
I can't say that to him/herの次に he is my dad/she is my momと言ったら親の立場を強調します。「親にだからこそ」の意味です。
仲が親密の友達の場合にはこのようです。
I am too close with him/her to tell him/her that.
I can't tell him/her that, he/she is my friend!
ご参考までに。
アメリカ合衆国 
