I think the parent killed the ko due to her repeating the 'Easy Laid-Back Days' song. Since she is a Deibu, I won't put it pass her. Goddamn karma Houdini.
I think that the Mother killed her daughter. In 4, I'm pretty sure that's still the mother based on the wording. Then in 5, my guess is that the Mother's lying to the daughter, and kills her by 6. Most likely because the daughter is "uneasy" for thinking that they need to work, but that's speculation on my part.
Two crackpot theories here: -They got kicked out of the clan, found a new place to live by themselves. Reimu learns to hunt and take care of herself. Somewhere during all this koReimu gets injured and dies. Reimu ignores this fact and pretend it didn't happen.
-They stay with the clan, the singing works and they get fed. If koReimu was the better singer, Reimu squashes her for being uneasy by singing too hard or something like that.
I favor the first one just because it's less horrible.
One notice mother reimu call herself reimu. Koyukkuri call herself reimyu. In panel 4 is mother. Koyukkuri never join her mother 's singing even once. Still don't crack anything yet.
Looking back at the original, I think the fourth panel is better translated as: Reimyu became disgusting. The proud Marisa's child Couldn't help becoming a Deibu.