My father made an Apple ID and doesn't remember the password

Hello, Apple's Support Team. I have a BIG problem.

Let me explain. I don't use the iPhone, yet, but my sister's do. My sister has used iPhone with not an Apple ID made by her name but my father's name about two years.

Without any talk with her, my father made an ID and he doesn't remember ANYTHING!

So this is >the problem<.

Since he doesn't know anything about the account(including a username, password, hints for the password, etc.), my sister cannot access to all the functions and services of the iPhone.

All I know about the ID created by my father is two things.

First, the ID's email

Second, the ID's name (found in iPhone's basic app-Setting)

Because I'm Korean high school student, my English composition will not be smooth. Please understand and help me to find my father's ID. I'll be waiting for your reply.

[Edited by Moderator]