So the sister abandons the brother at the park AFTER she realises he's knocked out, goes home to WAIT for the mother, both of them decide to go back to the park WITHOUT CALLING FOR MEDICAL ASSISTANCE! All of this taking at least 2 hours (see how there's some color in the sky when the sister pushes the brother but it's pitch black when she returns with the mom).
While the sister's actions could be explained by her being very young and ignorant (she's probably like 5-7 years old and probably couldn't lift/move her brother), the mom is EXTREMELY IRRESPONSIBLE! Her daughter whilst COVERED IN BLOOD tells her that her son is BADLY HURT (UNCONCIOUS even), and she decides to GO TO THE PARK instead of CALLING FOR MEDICAL HELP! Mind you the mum KNOWS that her daughter had been WAITING FOR HER to come home and she KNOWS her son is so BADLY HURT that he CAN'T COME HOME! She must assume then that a significant amount of time hss passed since her son got injured. So why wouldn't she call for medics immediately to cut down on the time he spent untreated and reduce the likelyhood that he'd die?
Mum is an utter idiot. Well the last chapter told us as much considering she thought it appropriate to barge in nude while her son was washing himself and then offer to help him.
Dog girl (prior manager) >= Neigbour >= Senpai > Sister >>>> Mum >= Childhood Friend.