Because unfortunately that's the real world is sometimes my dude. Adults are worse than children and humans are very impulsive. They do stupid things all the time. Saying this as an adult.
There's probably a bigger issue than that and they just use the kids as an excuse to cut ties
The fight between the kids lead to a fight between adults because the Mu family is basically jealous of Yunshen and his kids.
Their kids had been tested and shown to have no psionic powers. Gala and Garan are the only kids not tested so they're the last hope. Also Yunshen and his family usually keep to themselves and don't want the kids tested so the rest of the family felt like they were hiding something (which they were). It seems like the Mu family looks down on Yushen's family as well so for Gala and Garan to be the only kids with powers is kind of a thorn in their side. Despite that, it seems like kids are kind of seen as family property, especially if they have powers so even if the Mu family looks down and Yunshen's family, they still feel entitled to taking the kids and raising them because they are useful to them. Yunshen and his parents don't want to follow tradition and it pisses them off.
They cut their ties because of a difference of opinion. The opinion being that the Mu family should have Gala and Garan despite never really wanting them around or treating them well and Yunshen and his family giving a middle finger to that.
The old man felt disrespected because he's the patriarch of the family so he thinks his word should carry the most weight. The idea that a descendant would contradict him or rebel is an affront to him. He thinks when he speaks others should listen and do what he asks no questions asked.
I do think there are too many members of the Mu family. They seem not to care that Yunshen is family and only want to use him. It's kind of creepy and gross the way they obsess over him.
5 or 6 generations existing at once isn't implausible if there is longevity in a family.
Thank you all for taking efforts in sharing your views and facts. It's kinda scary to have such suffocating family like them. It would be better if Yunshen's immediate family would just shut their door to their other relatives. I actually find the story interesting, just as I read all the chapter until the current one. It's just that I vented out my frustrations here in the comment section because Yunshen got so many problems shot all at one hahahaha in addition to that are the annoying antagonists of his family. I just want to smack their head one by one so that they will leave Yunshen and his fam alone.
I really find some of manhuas to be exaggerated/overacted. They tend to make small problem into bigger one, and they are all too childish. Like for example, at the birthday party of that old man there, the great great grandfather. The fight was only from the kids, and it's all normal. But why does it lead to cutting ties with family?! Like wth, Garan and Gala is more mature than those old farts there. They could have just calmly ask what happened and try to look for evidence (such as cctv since its a very modern world) if they won't believe on the children's words.
Another thing is that super old guy, the head of Mu family I guess, he should also behave more calmly and properly infront of many people right? Like duuuhh he's acting like a spoiled kid. Aauurrghh!!!! I really hate the audacity and stupidity if the characters in manhuas.
Another thing, I Mu family is really confusing. Like how big that family is and how many generation present in there hahaha like there was 5 generation then hahahaha.