Proposed: When a faerie who cannot lie tells you that she cannot do something, she's probably not speaking metaphorically.
Theory: Molly literally cannot tell her parents that she's the Winter Lady.
In Cold Case, we learned that there are things Molly literally can't say or tell people, without a fair bargain in place. She tries to tell Carlos what she's doing in Unalaska, and it keeps coming out as Maeve-style subtlety-of-a-two-by-four flirting instead.
Similarly, when Harry asks her to tell him how she's handling being Winter Lady, she says: "Can't, literally. Faerie mystique and all that."
When he raises the subject of telling her parents: "I will," she said with a bit too much instant assurance. "You know. When I find a way to bring it up." Perhaps she means this literally as well -- she can't just bring up the subject and tell them, without some sort of pretext or bargain or way to make it her duty to tell them.
In Peace Talks, Harry cuts off her explanation fo why she hasn't told them, assuming that he understands. What if he doesn't? What if her explanation was going to be that it could harm the cause of Winter to have Michael angry, and she literally can't take that risk without a benefit to Winter to balance the scales. Through the rest of the conversation, she says things like: "I can't," (italics in the orignal), "It hurts," "I can't," "I can't," and "I can't ask you to do that."
Remember that she can't lie. When she says that she can't do something, we should assume that she's being completely literal.
It's a horrific conversation when you parse it that way -- she's weeping and begging him to help her and he just thinks she needs a hug and some emotional support.
