It is easy to underestimate the level of attachment you would have with a sibling. Because being in the same household practically means you are seeing the same person every single day.
You don't 'miss' someone you have just seen in the past 12 hours.
But once that someone is gone for a lot longer, the feelings of attachment really get to your head.
My sister just got married 2 days ago, and knowing that another one of my siblings will be moving on to start their own family unit...it left me a little bit more lonely.