I would not make a "big" deal about it. In fact, we didn't. We only have 2 kids, but our oldest was not recommended for testing, and I never pressed for it. With our youngest, I did press for it, though they would have gotten around to it with her eventually anyway-I just wanted her to be able to take advantage of the opportunities as soon as possible.
I would respond this way, to your specific question: Would you tell your daughter that you were excited for getting into a "special" program if the "special" was "special needs" instead of "gifted"? I say that, again, as the parent of 2 kids: one in the gifted program and one not. Yes, I am very excited that my daughter is receiving the services that will help her expand her knowledge and skills and challenge her in the most appropriate way. But, I do NOT praise her for doing abundantly well on academic matters just because she makes 100's. She needs to have been challenged and worked hard at something to garner praise from me. Believe me, your daughter KNOWS when she is receiving praise for something that she doesn't truly deserve it. Just being the best at something isn't necessarily worthy of praise, as difficult and confusing as that can sound. And I would be very cautious about WHAT you are giving her praise for. For my daughter, it is for thinking outside the box, doing generous things for others, her attitude being positive, working hard/doing her best at ANYthing(chores, playing, whatever, not necessarily academics). Rather than praising her for reading books that "should" be beyond her level, I ask her questions about the book itself and what she enjoyed or disliked about it.
Those sorts of interaction will be beneficial to her AND to your older daughter.
Find things to praise your older daughter for as well. When you know she struggles with something (talking in class?) then praise her when you notice that she has refrained from doing it as often as usual, or for some short finite period of time. For my son (who is the eldest of our two, and the one not recognized as "gifted") it wasn't that hard to find. It isn't academics, though he carries a B average usually. He is a very sensitive soul, who seems to innately know how to engage people and set people at ease in uncomfortable situations. He is a people person. He is gentle and helpful with younger kids. And he is very funny. So when I see him go out of his way to be nice to a stranger or whatever, I make sure to "notice". It helps him to recognize his "talents" that academic circles might not count as talent. But they are talents. He even became very engaged in his science class the 3rd 9 weeks of this past year (he was in 7th grade) and helped "tutor" some of the other kids struggling with the material he had down pat. His teacher told me about it. I wouldn't have known otherwise.
So, back to your question. I would NOT make a "big" deal. I would inform her (once you have the details of how being part of the "gifted program" will affect her daily activities with school) what will happen. (For our daughter/school... she goes to an entirely different school for an entire school day one day per week. And she is not required to make up missed assignments, though if they have a test-usually the teacher does tests on other days--she makes it up the next day or whatever). It is not something your daughter has "accomplished" and your older daughter did not "accomplish". It is just placement in the program that will help your daughter learn to the best of her potential.
Hope this helps!
**After your 2nd edit:
I would not tell her much, because you don't know much yet. My daughter is the type that needs details, otherwise it just causes anxiety for her. So, if she asks again, then tell her "Yes, you were accepted. They will provide us with more information about how that will impact your school day, but we won't get that information until school is about to start" Or something along those lines. OR, just tell her she won't know until open house, unless the NOT knowing is causing her more anxiety.
For now, letting her know that she will be in the program will probably be all you CAN tell her. How that translates into the routine of her school day is something that you will need to talk with her about when you have that information. And something that not knowing might cause anxiety for her.
So a head's up to watch for that....