Elatha: Little moon, will you come to me?  
Luna: Yes Pappa  
Elatha: I… I have done something terrible  
Luna: What is it?  
Elatha: My brother, the King, ordered me to take in a witch, but in my attempt to do so I used too much force. With her dying breath, she cursed not me, but you my young one.  
Luna: Pappa..?  
Elatha: Promise me something.  
Luna: Anything pappa  
Elatha: It is a lot for me to ask, when it is my fault you have this curse now, but until it is broken, do not have children. I don’t want anymore innocents to be inflicted because of me.  
Luna: How do we break this curse then?  
Elatha: The witch must break it herself. I will find a way to do that, don’t worry yourself. Promise me, that you will not let anyone else be cursed because of my mistake.  
Luna: I promise pappa!
 

 
Setting up some history for a Future Gen, wink wonk  
Just to be clear, Celestia and Luna are biologically cousins, but Luna was adopted by her Uncle which added to her isolation from Celestia.  
And the curse mentioned is the one that allowed Luna to change into Nightmare Moon.