If I recall, you've adopted your niece's child, right? If the news she shared is related to HER exprience with adoption, I'm not sure why you feel betrayed. She has a right to share her story with whomever she chooses even if you had an agreement not to. If the news she shared is YOUR personal news, unrelated to her, then yes, I'd be a little miffed. Is your adoption supposed to be a secret or something? If so, why? Perhaps she initially felt like she didn't want to share but now feels it's beneficial to her emotional well-being to have things be out in the open.
If you feel that strongly about it, perhaps it's best not to see eachother right now - but long-term, this is someone you will always have a relationship with and it's probably best to sit down, face to face and discuss your feelings. It'd be a shame for her not to see the child while you're in town. Is there a neutral third-party that you trust to make sure that happens outside your presence?