Even two dogs from the same litter can forget the meaning of "brotherly love" from time to time. Some dogs are just competitive by nature, while others feel compelled to fight over resources, like food, mates or even your attention. By determining what your dogs are fighting about, you can address the issue and put an end to it.
Neuter your dogs. Male dogs, siblings or not, are more aggressive when they're intact. Neutering not only removes the motivation to fight over potential mates, but also reigns in the hormones that can compel them to fight in general.
Give your dogs equal attention. Most of the time, when two sibling dogs fight and sexual hormones aren't the culprit, they are fighting for dominance in the pack -- and to your dogs, your attention is a reward for dominance. For this reason, coddling a dog that loses a fight is confusing to your dogs, because the winner of the fight should be receiving your attention. If you need to spend separate but equal time with your dogs, so be it -- leave one at home while you walk with the other, or play with one while the other takes a nap. Whatever your strategy, give them equal time to remove the compulsion to compete for you.
Separate the dogs before you give them anything. For example, feed them in different rooms at the same time every day, and only give them toys to play with when they are in separate rooms. This doesn't mean that they always have to be apart -- just don't give them anything material to fight over when they are spending time together.
Give both of the dogs plenty of time and opportunity to socialize with other dogs. By meeting and playing with other dogs, both on walks and through scheduled play dates, they gradually lose the sense of fear that may compel them to fight even each other.
Stop fights before they occur by watching the dogs' body language. If you see that a fight may be coming -- look for signs like stiff movements and bared teeth -- separate the dogs, both from each other and from you. They'll learn that getting ready to fight doesn't get them anything they want.
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