If there’s one person who has to be at my future wedding and vice versa, it’s my younger sister. We have discussed many times whether it is best practice to choose your sister as your Maid of Honor because it is more diplomatic than picking a favorite from your other friends. At this point (I am painfully single), she has the spot in the bag. 

Your sisters and brothers are the friends you’ve known the longest. They are the only people who know exactly how your parents are and can understand where you’re coming from without explaining it. If you’re lucky, they are the family members who will be in your life the longest: probably longer than your parents and grandparents ever will. I would consider it a tragedy of unspeakable sorrow not to have my sister be a huge part of my wedding day. But hey, that’s just me.