Genes are blends of traits. Combinations formed randomly. Good combinations survive, bad combinations do not. We, as humans, do not decide, cannot decide what is a good combination.
Born of the same parentage does not mean similar personalities, looks, likes, tastes, etc. All it means is same "pool" of genes came to work initially. Chance decides the rest.
As people grow the choices we have are with whom shall we share our days, whom shall we share our secrets, whom we shall procreate. As adults this may not be a true thought process. It may be more of an unseen force if you will; something that draws two sets of genes together. I believe good combinations are not thought out. They just happen. Fate.
I think this same force plays into the friendships that we form. Something draws us to certain individuals. Unseen oras. Magnetism.
With siblings the combination of genes may cause all to be too similar, even though you can't see it. Like two magnets repelling each other when the similar sides are put together. Friendships are like polar opposites. Once they connect, they are hard to pull apart. Family relationships may always push apart. More effort has to be put into family relationships if they are to work because of this pushing apart. That is IF one wants them to work after the common bond (parents) are no longer on Earth.
Friendships are easier. Friendships can be more rewarding. But, that blood connection is harder to severe. My parents had this Pollyanna gene combination that I think I alone of the siblings inherited, it commonly shows itself to us as guilt. Guilt resides in broken family ties, not in broken friendships. I wish scientist could locate my Pollyanna gene and cut it out. This gene causes me to worry about my relationship with my siblings more than it causes my siblings to worry about their relationship with me.
My New Year resolution will be to focus on those things that give me the greatest pleasure. That is my husband, my children and my friends. And probably my cats too! :), and the birds at my feeders, and nature, and sunrises, and helping others, and ...