Why does the society "clustered individuals" expect siblings to get along?

A society is made of individuals clustered in one space by beliefs. Siblings that have no common
belief will never get along. A bond between siblings is developed from youth and parents making sure they don't use their kids as tools to fight each other. It sure takes some divine intervention to keep siblings together. Usually this should come from the eldest but sometimes the youngest possesses the gift of wisdom.

When bringing up kids the parents should cultivate respect and free will to express oneself so that mine, yours can become ours. Using the quote from the bible Luke 8:21 on how Jesus addressed the call of his mother and brethren and could not come at him for the press.
20 And it was told him by certain which said, Thy mother and thy brethren stand without, desiring to see thee. 21 And he answered and said unto them, My mother and my brethren are these which hear the word of God, and do it.

To hear the word of God in my view would be "those who have the will to share views and draw understanding for the common good and implement it". One could say that the path that Jesus took was scary not just to the public but his family as well so they usually didn't gather together to talk. I use the stories in the bible to draw perspectives of what may be happening in families where siblings do not get along and usually because an understanding hasn't been developed or a kid decided to separate from the family due to varying value systems that can't meet. Principles and beliefs separates or brings people together. "Is family those who you are born with or who you bond with"? Loaded question hmmm? My kids friends are my kids thats a smarter way to take care of my kids indirectly.