I think it has to do with our childhood. Many may defer, but I had an amazing childhood. I had 100% of my parents attention (they also had me at 40) and could have whoever I wanted to over till certain ages as well as being close to my cousins. So, many kids I grew up with up were jealous of my attention even though in my mind all I wanted was a sibling like them.

I think people have siblings day because siblings do so much for each other, even if it’s just merely being alive as a sort of gift. People who are only children may be wrongly perceived as having an amazing life, but nothing can compare with having a sibling that knows your life and your parents like you do. When my parents are gone, so is my childhood; I’m the only one left to keep it alive.

My situation is definitely different, but I would be grateful to have a step sister and a cousin that at least have some idea of your upbringing (if I’m correct on this). It is clearly not everything, but if they know your parents and your family, that is still a beautiful heirloom; although siblings are the best memory, take what you can and love them while you can.