This is how I think the Shadowheart reveal should go, with other options, of course, but the option to say some of these things should be present.

But that wasn't what caught their attention. Shadowheart was fixed on a damaged old statue before them. Suddenly, pain rippled across her features and she held her left hand up before her, looking at it as if it was alive. The moment seemed to pass, and a look of terror seized her. She looked up at Vexir, her eyes were like that of a frightened child looking to her mother for help. "We... we should keep moving," was all she could say. Then she tried to harden her features.
"What just happened?' asked Vexir, concerned.
The cleric tried to smile, pretending as if she was perfectly fine. "Nothing," she obviously lied. "A trick of the light. But something tells me there might be a solution to some of our problems hiding in this wilderness."
Vexir’s expression softened. "If there's something else you want to say, I'm listening," she told her.
Shadowheart sighed. "There's signs there was a conflict around here, some time ago. Conflict needs opposing sides. Whoever they were, they must've had resources. A little investigating could turn up something of interest. Let's go."
Vexir put her hands on her hips. She made it obvious she wasn't budging until Shadowheart laid everything she had to say out in the open. "Listen," she said. "It's obvious to everyone by now that you're not a cleric of... whoever you said you were a cleric of before. All this talk of conflict and resources, that isn't what you're really getting at. Just speak plainly. We're all in this together at this point. There's more to this. What aren't you telling me?"
Shadowheart seemed both angry and relieved at the same time. "You're not going to let this go, are you? Fine. I worship Shar, the Mistress of the Night." She then pulled out her holy symbol and affixed it to a headband of metal. When she put it on her head, the symbol displayed a dark circle at the center of her forehead. Now that they knew about it, she displayed it proudly.
Astarion snickered. "Really? I never would have guessed." It was obvious that he was being sarcastic.
Shadowheart ignored him. She was finally confessing her biggest secret, and she now needed to get it off her chest completely. "The destruction of this Selune statue looks like the handiwork of my brothers and sisters. Now that you have the truth, please don't make a big fuss about it."