Whether these twins are good and evil, quiet and loud, book-smart and street-smart, they’re in some important way opposite to each other. Of course, these twins are almost always identical, as well.
Why this can be bad: While there are definitely twins who embody this idea, writers can sometimes go completely overboard with this to the point that the twins don’t even seem related. Another character might even comment on this because it’s so blatant. When you take this to too much of an extreme, your readers will find it hard to believe that these two ever even existed in the same universe.
How you can fix it: Nature versus nurture is a big debate, but psychological studies have found that genetics play a heavy role in personality. While any family can have a diverse array of personality, there is always some uniting factor. While we might not want to admit it, we’ve all had a moment where we do something and realize how identical it is to our parents or siblings. Whether raised together or not, we have lean towards the personality traits of our immediate family, which means our siblings, especially genetically-identical twins, have a high level of similarity. This isn’t to say that your siblings can’t actively fight it or try to avoid it, but it’s important to keep in mind that they will have a decent amount of similarities.
Bottom Line: Remember that your twins are related, and if they’ve grown up together, then they’ll still have similarities between them.