What is the meaning of - le voy a decir a mi hermana cuando llega mi papa?

if the sentence you posted is correct t means "I'm telling my sister the time my father arrives", but I find it difficult to imagine such context.

I believe there's a mistake in the sentence and it should say "le voy a decir a mi hermana cuando llegue mi papá ": "I'm telling my sister when my father arrives". Unlike English, a time clause in future mustn't be in present, but in subjunctive.