I think the 'witch' was the twins' mother who travelled around as in previous chapter, her ears was more pointy (she doesn't seem human) and she seems to be the only one looking for them. When they got to the 'water village', concidently, drought started to happen. People began to mistrust the twins and the mother, attracting merchants to the area.
What's confirm is that the woman met them first before the merchant as they look younger. I can also guesd that the twins were already immortal before the merchant. It may be due to what their bloodline.
And I believe that the 'witch' was kind to began with since the twins always seem reminded of her when someone shows an action of kindness.
My theory is that one of the Merchants, 'hero', found the twins and told them about bread. I think he purposely place poison in it and gave it to them who would give it to the 'witch' as children would to let their mother see something first, causing the mother to die.
This would mean the so-called villain, 'Witch', becomes the hero for dying for them and the hero, 'merchant', becomes the villain for attempting to kill the merchant and killed the witch. And how much they want to be the so-called villain unlike the merchant.
I don't think the merchant die of poison personally as poison will only cause people to die of high fevering, not bleeding. In previous few chapters, the man who got poison by twigs didn't cough blood so I am guessing the merchant dies due to the twins killing or some accident happen to him while he went to search for them as he felt gulity of killing the mother for the sake of fame.
As the twins' form of revenge, they licked some of his blood to become 'eternity'. I intepret it as, "You're gonna live forever with us, you can't die yet." They probably wanted to feed the same poison he gave their mother, to him inside them. Since their mother died due to it, they are sacrificing themselves to kill him in them the same way he did. This also associate with the death card, they are willing to die.
As such, the books with food mixing with poison etc written by one of them is them trying to find the same poison and recording it. They travelled around to find the poison.
I think they become bread makers as the merchant mention, "It's gonna be the most famous thing" and they are children with no knowledge of outside world things as they asked what was bread.
Midway, they found the child who was poisoned by twigs and gave a bread with their some blood in it to him. If it cures, it's not the same poison as the one that killed the mother.
All in all, they went back to the same village and began researching the surroundings. In the end, they may had found the poison and built their own coffin to let them stay with their mother, the creek.
However, this is just my theory so..