Trololo said: There is a translated version too, y'know.
The fault is more on the mother's hands, actually. For all her good intentions, the mother Marisa never once properly explains to the hat marisa that the shell make it so that Shell marisa NEEDS the extra care, and it's not just "pampering" shell marisa and ignoring hat marisa as much as it is giving shell marisa the care she needs.
Patchouli even warns the mother that this could happen and tells her to talk to hat marisa about it, but the mother never does.
Thus you're left with an insanely jealous sister with a burning inferiority complex, which leads to this. If the mother had explained things to hat marisa properly, she would have been much nicer to her sister.
Also, even though hat marisa intends to upset her sister, she never means for shell marisa to die. She is equally shocked and dismayed to find her sister dead when she is woken up.
Trololo said: Do you think so? Well I better not show you where the lemonade is made! Sweet lemonade, sweet lemonade, Sweet Lemonade, Sweet sweet lemonade.