Not all family members could be friends.
Kids in similar ages grow older with each other, stay together in the pictures, witness high and low moments of each other, and never doubt why they have to share the same pieces of life, including school, work, marriage, next-generation. That's the power of blood between brothers and sisters.
My father, the firstborn boy in a traditional south family which means he is born with the responsibility to be the leader of his younger brother and sister. There is an invisible wall called "rank", and my father also follows such a pattern while raising his children. The result is that we have never told to be friends, but to be the ranked competitors for the distribution of limited resources, like more parents' care, further education chances.
However, my second sister and I, we are born to be sisters, but choose to be friends. When we get older, we don't take blood as the guarantee of endless forgiveness, unconditional love or ranked responsibility. Though we share the same parents and childhood memory, that's the truth and just the basement of our relationship, beyond that, there is the distance of complete independence.