The reason we need siblings is not because they share our genes and are our closest blood relations. It is not even the sentimental Bollywoodish “we are born of the same mother in the same womb” dialogue. It is not because our brothers and sisters are the storehouse of our childhood memories and will probably be our link for life. The reason we need our siblings is because they are the only people we can talk to about our parents and expect complete understanding…
Your sibling is the only person who will understand your family’s quirks and dysfunctionalities and accept them without judgement. Even your spouse, however tolerant, will take time to buy into your family’s very particular traits: the penchant for gift-giving; the focus on food; the pattern of spending. All this can seem weird to someone who wasn’t born into it. A sibling on the other hand will both understand and do the eye-roll.