Something like:

Dear Bio-Dad:

I've recently moved back to Michigan and since I last wrote you, I have gotten married and have had a daughter. I would very much like to contact Sister, but wanted to give you the opportunity to break the news, as it were, to her about having a half-sister. I will be sending a letter to her at the beginning of March.

Hope all is well with you,

Signed,
Daughter you refuse to acknowledge and now you don't get to know your totally kick-ass granddaughter, you halfwit.

I would change the wording of "I would very much like" into something more definitive to make it clear that he does not have control over your actions, only his. Like, "I am going to."

(Whoops, hit post early.)

But other than that, I think it sounds great.

Is there a reason you'd be waiting till March, Aimee?

I think I'm about to eat a television remote and a couple of pens.

Funny, I think Mr. Peabody is posting for Ginger!

Is there a reason you'd be waiting till March, Aimee?

I just picked March randomly, fwiw.

Oh wow Aimee - big day! Good luck, and I hope it all turns out for the best.

It seems like about a month would be a good amount of time to sit with the information and give him a chance to let his other daughter know if that's what he wants to do.

Aimee, what will you do if he responds that she already knows about you and doesn't want to be contacted?

Aimee, what will you do if he responds that she already knows about you and doesn't want to be contacted?

I wouldn't put any faith in that at all, really. Either way, I'd go ahead with the letter.

I think that's one reason I'd hesitate to notify bio-dad. Not only might he lie, he also might decide to paint a nasty picture of you or your mom and taint the whole process. I can see the other side of the argument too, though. Notifying him first might make things easier in the long run. Tricky, really, the whole thing. Tons of ~ma as you figure it out, hon.

How old is Sister?