Can you imagine how much worse it would be if she found out from some other method? As an emotional teenager? Or as an adult? The level of betrayal would be massive.

You've totally done the right thing telling her now. She'll take time and process and just make sure you're there to talk to. Be aware at some point she may show interest in her bio father and that's okay - it is in no way a rejection of her Dad. It's a natural, normal curiosity and part of the processing. Just be honest. Age-appropriate obviously but really really honest. She'll sense if you're hiding details or prevaricating and it'll make her less accepting herself of the facts. Open, present and matter-of-fact is the way to go here.
You'll all be fine - and better for it!!