Dear Mrs Tonks Andromeda,
I'm sure you weren't expecting a letter from me, were you? Frankly, I never expected to write to you myself.
Well, I want to start by thanking you for everything you've done for Sirius over the last year. Longer, really. I can't imagine who he'd be today without you to encourage him.
Which leads me into the main part of my letter: I know that he's told you all about us. Us, that still feels odd It's a little weird, I guess; I haven't told my own parents yet, so knowing that some kind of parental figure knows is...well, a little weird (forgive my lack of vocabulary). But I'm not upset or anything! It's good that Sirius can talk to someone easily about us. There's that "us" again... I've got a cousin in Italy who's been very helpful, so I know how much of a comfort it can be.
I guess the only other thing I need to say is that before anything else, Sirius is my friend, and I'd sooner die than hurt him. Just so you know, I mean.
Thank you again, and I hope you don't think any of this is weird.
Sincerely,
PS: By the way, your present to him was BRILLIANT. Truly amazing!