Dear Princess Celestia
I learned something very important. I'm not sure exactly when, but I know I know it now. Family can be complicated. You don't get to choose your family, and while you can walk away from them, they still are your family. Friends you choose, family you have, even if you don't like them. But I think good friends and a happy family work the same way. Even if you don't always get along, even if there are problems, in the end you're there for each other. Whether it's a kind word, a little help, or even a willingness to sacrifice yourself for their safety.
Family like friendship also might not make any sense to anypony else. Why does she tolerate him? What does that stallion have to offer her? And countless other questions, but from the inside it's obvious. From the inside a good family is full of love and happiness, and everything else doesn't matter.
Finally I learned that a pony can be terrified to make friends because they fear the loss that becoming too attached can bring. With family this can sting all the worse, but it's still worth it. Mom started by offering her hoof in friendship, and over time she slowly got him to let her in. Slowly dad got to know her, and of course the more he knew her, the more he loved her. So now he's left behind all that pain and found a family. Found happiness. I have my dad back, and mom doesn't doubt anymore.
So what I think is the power of friendship is what builds a good family. There are other important things that make it something more, but the idea is the same. The love is still there, if a little different. And that means everything.
Your loyal subject, Time Filly Rosemary "Dinky" Doo
P.S. If I got confused and you have no idea who I am or why I sent you this letter just keep it in storage. It will make sense eventually.