Nathaniel asked why he never brought it up before, and Merlin said that the method is dangerous and if the message is intercepted, it could reveal their location to nearby enemies. Aside from that, the spell needed a "perfect" blood relative on both the sending and receiving ends. Nara mentioned me and Elizabeth, but I shut down the idea by telling them that we're only half-sisters, which was news to everyone.
George and Nathaniel both questioned me, and I told them how we have different mothers. They didn't believe me until Merlin pulled out a feather and cut my hand. He dripped blood onto it and said an incantation as well as Elizebeth's name. The feather floated in the air before just fizzing out like a flame. He said that without a "perfect" receiver, the message will never be sent.
Everyone was stunned and confused at the revelation, but it put us at a disadvantage, since no one else had any relatives who could help us. Except Joan. Joan reminded us that her older brother was the one who helped us from the start. In addition to that, he was her "perfect" sibling. In the end, we decided to wait until we got to Ceres before sending out the message since we had no way of knowing if he and Sarah even arrived.
Merlin gave Joan the feather and he said an incantation on it. All she had to do was drop her blood onto it and say her brother's name. After she did, the spell would do everything else. Donatello also told us that the base of the mountain was a half-a-day walk and that we'd be there a little while before dusk if we left in the morning. I went to bed excited since I would witness the most powerful warlock in the world blow a hole through a mountain.
My excitement turned to curiosity as I was talking to Kevin in the flower field. I decided to ask him about Merlin's past.