Most sibs aren't alike because each one has to differentiate himself to form his own sense of self-identity. It's harder to do and requires more effort when they are the same gender. Some become best friends and others can't stand each other and everything inbetween. It is the ones who can't stand each other which interests me. Here are some patterns I have seen.
One is the sister who suffers from the low self-esteem, constantly comparing herself with her more accomplished, more talented, prettier and/or favored sister. She works at annoying that sister, without even understanding why, to get an irritable reaction, thus proving to herself that she's nicer and the supposedly wonderful sister is nasty and rejecting. Hopefully, that sister can see through the manipulation and choose not to play.
Another common pattern is the brother who rebels vs the easy-going, subservient brother. Sometimes, which brother is which switches, just as long as someone takes each role. They are both playing two sides of the same coin, often portraying the tension between the parents with the rebellion and tempering it for the sake of some peace and love with the subservience. Only so much turmoil is tolerable. Only so much sweetness makes sense to growing boys. They are a team, whether they know it or not. If only the parents can see through the game and make peace with each other, the children can stop playing it.
Then, there's the sibling pair that pressures each other to higher and higher goals, competing with each other and the world, whether in sports, academics, crime or fame. They are high achievers, whether for good or bad but not happy because nothing is ever enough. They are trying to reach some unattainable family status, maybe to make up for past generations or to keep up with them. Think Venus and Serena or the Kennedy's, the famous football families or the Harvard legacy families. Under all that pressure, there's both appreciation for the achievement they attain and anger about the pressure. They aren't sure whether to be angry at the parents, themselves or each other and vacillate.
There are other patterns and variations for sure. In a truly loving family, at best there's love and getting along among siblings but even good parents can't always protect against sibling rivalry many times.