If there are two objects in a sentence, the normal word order is:
I must give my sister .
I gave Mandy .
I gave her .
I must give to her.
If we want to emphasize the person, we put the person after the . Then we use the preposition to.
Let's write to John in California.
Using the words describe, explain, report, say - we always put the person after the . (preposition to)
I explain to you.