It's always nice to see yukkuri with enough sense to punish their children for being shitheads. Even if the parents themselves ultimately don't prove particularly laudable overall...
I'd say they were pretty good parents. They even took the risk of trying to cure little Marisa despite the risk of the mold spreading. I'd say it comes down to how much they told little Marisa about mold and why it was quarantined, which I know nothing about since I can't read Moon. But at the very least it knew that mold wasn't pleasant, having suffered it itself, and it spread it knowingly anyway, so it's barely better than a shithead at best.
I think you're too quick to pull out the shithead label.
Maricha is just a ko being a ko. Even if it knows, knowing is not the same thing as understanding. Her parents understand the dangers through experience. Yukkuri are very social creatures and they aren't going to take being asked to die in a hole alone nicely.
This would be similar to an infected kid during the dark ages of the black plague. It is still going to want to live its life.
But it wasn't told to die in a hole, just to stick in a corner of the house until it's recovered. Komarisa was sick, knew it was sick, and knew Koreimu was sick already. Then, just because it was lonely or jealous, it decided to infect its sibling further. It reminds me of Y's previous story, in which the Reimu rapes its younger siblings out of jealousy. Perhaps not a shithead like the ones that do nothing but insult and openly treats everything as shit, but rather a complete lack of empathy and sympathy. Perhaps it could grow out of it, but yukkuri tend to be portrayed as more or less set in their ways from birth. Comes down to author and story canon, I guess.
maricha was jealous for skinship w/ parents, but it's not like she infect the mold w/ intent to kill. Just doesn't know, unaware of it's lethality. Still scum.
in canon and ankos the knowledge/treatment of 'mold' kinda spotty from yu to yu.
For that little one too...fluttercrapshootsputcrapshootspin spinGyupicchi!Mugyu-Stop no ze, Reimu.THUDspin spinNo! This is revenge for little one!Mubu-!?Dis...!!Why are little sis's eye-eyes open even though little sis is sleeping?Take a good look no ze, little one's little sister is already dying no ze....!Little sister murderer!!Yue?