Singularity question

This book, where the main characters are twins, has a lot to do with sibling relationships and sibling psychology and I'm asking, how can you explain sibling relationships? The bond between brothers and sisters. In my opinion, sibling relationships are hard to explain, there are many different types, but I think no matter what, they always love each other. Though they may fight, and tell everyone how much they hate their siblings; they will always love them deep down, and wouldn't give anything to lose them.