Well in the previous page the mother reimu showed her about poisonous mushrooms and stuff, maybe this time they throwed her everything that was poisonous?
Basically Patchouli told Mother Marisa to take extra care of Shell out of being special and also needing more protection.
Cue for a small misunderstanding on which big sister marisa bumps by herself against Shell and hurt herself against the shell, and then tries to attack her, from that day onward the mother separated them and acter way more overprotective on Shell.
In this case is 100% "new kid > old kid" out of being new.