So since the youngest brother is obviously the villain of this arc (along with his mum); I'm going to bet that all of this aggression towards his sisters and not mum is due to jealousy or something. His brother provoked him by saying Coco used to protect him.
Edit - Welp .... I made the mistake of reading the comments on the raw's site. Even the reader's in Japan weren't thrilled with the series.
"How did this become a battle manga? Wasnt the main character a carpenter?" And "you can tell when this was cancelled; they tried to still use the same ending but skip 60% of the story."