Let me do the "useless coworker in a meeting" schtick and collect our thoughts.
1. Bro is a fucking dumbfuck. No matter what his reason was to do this (tsundere who loves his little brother and wants to test the fiancee OR abusive asshole) he is an idiot who is actively causing a huge PR disaster for his house. Attacking a rival house's head for.. what exactly? Visiting her fiance??
2. MC is a complete doormat, who is happily accepting his role as third son and being abused with not a complaint. Dude atleast have the spine to be upset your whole family hates you. What is this milktoast "I understand

Its okay, this is just how life is

)"
3. Fiance should have thoroughly slapped the brother into the ground. Just deal with it. You're at the top here and can just say you were wrongfully attacked. Self-defence yourself and show this family you aren't a doormat like your useless himbo fiance.
4. My god this manga's plot is F-tier. Like the characters and plot and all HAS potential, but its lost under horrid writing that is doing no favors to anyone. Everyone looks like an idiot.
In conclusion, read this for the sick pleasure of watching a carcrash in motion. Do not read this for spite, its not worth it.
Thank you for translation however, I personally do enjoy stuff like this where its just unashamedly garbage. Its a nice balancer for more quality stuff. Keeps me grounded, ya know. xD