Eventually, Mother did demand the truth from me, though granted me the request of allowing us to have the conversation at the House rather than it being just me on Olympus. I got back and immediately went and cried to Hades; nothing scared me more than what would happen if they found out the truth, but she wouldn't have settled for another half-truth.
When the time came and everyone arrived, I honestly wasn't sure what would happen, because Zeus was liable to smite us for exposing him and Mother might have for the lies, let alone how any of the others would react. If any one of them took it poorly, no amount of ambrosia would have saved us. Zagreus cut in before Hades and I even started, to give a condition to Mother, and I think I almost died then and there; he asked her to let me stay at home more if we told them the truth, and when I went to lead him from the room to have a conversation, Mother stopped me and... agreed to his terms? I wonder if the fact it came from him is why, because I knew she would never have if Hades or I asked, particularly without an explanation first. We told them all — Zeus ended up storming off — but Mother just quietly waited for us to finish, not once interrupting, not showing any anger on her face at all. Nobody else dared say a word before she did. I don't know how long we waited for her, but the first words were "I am so sorry", and I was totally taken aback.
Of all the things I expected my mother to potentially say, "I'm sorry" wasn't on that list.