C.,
Hi. Thanks for sharing your story. First of all, I'd like to ask, does Ann know that you know? If so, maybe you could talk to her about it, in a supportive and nonjudgemental way, but without pressuring her to say anything.
This may help her feel more relaxed about your family knowing and that you don't judge her. But I think once you're there, you'll have a better sense than anyone who could advise you from afar about whether to even talk to her or not (without making her uncomfortable).
If Ann doesn't know that you know, I would say, just let the two girls talk and she may open up to your daughter naturally. This would probably help her to feel more safe emotionally, and that her confidentiality is respected.
Also, I would say, it may be helpful to explore your own feelings and why you want your daughter to know. Perhaps journaling about this may help you realize something deeper than what appears on the surface.
Hope this was helpful.
S.