Is there a special event coming up?  Possibly your daughter's birthday, your marriage anniversary, or family reunion?  That would be a great time to bring this information to your daughter.  I recommend telling her sooner than later. If she discovers this information on her own, it will be more difficult to explain.

I recommend a book called Chelsea's Tree. It's a very precious story that describes a grandmother coming to visit her grandchild with a package of photos. She uses the photos to introduce her granddaughter to her blended family (in many different states, etc.)  

The book allows for you to paste photos of family members in it, and describe who they are and how they are related to the child. The book would be a great way to teach her, in a positive way, that families are a little more complicated than she realized, and she has family members in other places.

It's important to reassure her that you love her, and nothing has changed in your relationship.  Also be prepared for her to want to meet her half-brother.  I wish you the very best.

Kindest Regards,
Shirley

Shirley Cress Dudley, MA LPC
Blended Family Expert
Founder of The Blended and Step Family Resource Center