Siblings

From Pamela Reid's Dog In Sight

..neutered males are more likely to squabble than  spayed females; however, when females fight they re mor likely to inflect injury. And there's some suggestion that females may be less likely to get along after a serious fight than males. 

From ComPETability: Solving Behavior Problems In Your Multi-Cat Household by Amy Shojai

The worst aggressions occur between same-sex pairings, so pick a new cat of the opposite gender.

From ComPETability: Solving Behavior Problems In Your Cat-Dog Household by Amy Shojai

Some of the worst aggression, though, occurs between same-sex pairings with female-on-female the worst. Neutering everybody will help even the playing field, and solve many problems before they begin.

From Behavior Problems in Dogs by William E. Campbell

To avoid such fights, it is best not to obtain littermates of the same sex, particularly those that appear competitive within the litter.

From Behavior Problems in Dogs by William E. Campbell

Fighting between canine members of a household usually involves dogs of the same sex, often littermates.

From The Culture Clash: A Revolutionary New Way to Understanding the Relationship Between Humans and Domestic Dogs by Jean Donaldson

Double dogs also frequently bond more strongly to each other than to anyone else, especially if the two dogs are littermates.

From Dog Behavior: An Owner's Guide to a Happy Healthy Pet by Ian Dunbar

Most dogs have disagreements and arguments from time to time, especially two male littermates living together,

From The Dog Vinci Code: Unlock the Secrets to Training Your Dog by John Rogerson

puppy has been kept with its littermates or another dog for far too long, so it learns good social behaviour around other dogs but humans are a mystery to it because it does not live in a world with much human contact.