Being super close in age, I drag down fist fought with my older brother for years. I was obsessed with my baby brother, so fighting wasn't an issue there. But a lot of my friends, especially ones with sisters, fought incessantly with the older or younger sibling their entire childhood, and it wasn't until they both became adults that they actually started to like each other. It could be because their sibling was an asshole. Or there could be more to it than that. Well IT DOESN'T HAVE TO BE THAT WAY. Ask Dr. Laura Markham.
Peaceful Parenting, Happy Siblings: How to Stop the Fighting and Raise Friends for Life is a parenting guide to help you build strong connections with all of your children, to eliminate sibling rivalry and the idea that you love anyone better than anyone else (even though you probably do). What I found especially interesting? Getting rid of apologies. Kids (and adults) think that saying their sorry can fix anything terrible they have done. Well, it can't. So encourage your children to problem solve and "repair" the situation to the best of their ability. This book is geared toward families with young children (for example, adding a new baby to the family), so keep that in mind. Also keep in mind that Dr. Markham is a parenting expert with other books that could fit your family right now.