"...who her that told her that one of her children..." did the first 'who her' throw anyone else off?

Everyone keeps secrets at one time or another. With multiple kids, its a good guess that someone is keeping something quiet. Kind of how the psychic thing seems to work, play the odds and read reactions.

"Mom, the truth is...that under all of these clothes...I'm naked. Please don't be angry ill never do it again."

I feel sorry for you, OP. These "psychics" should be taken lightly...or just not visited at all. This will probably be a self-fulfilling prophecy as you and your siblings could start sneaking out or breaking your punishments.

You're an alien. I know. Tell her that the dark secret is knowing what she did last night. She'll be insecure.

Mom, the "dark secret" is psychics are con artists.

Just pee in your bed and tell your mom that your "dark secret" in that you are a bed-wetter. You'll be good to go and get a new mattress to boot.

Make up a secret. Something dark but not necessarily grounding worthy

Tell her you are actually catwoman.

"Mom, the truth is...I dont actually eat all of my vegetables. I feed them to the dog..."