The fantasy genre in general is known for one excellent trait in particular; its great ability to up the stakes of the story by revealing a game changing twist of some kind. Whether it be one of the main cast members turning out to be the villain, a character finding out they were adopted, or a betrayal by one of the characters, eight times out of ten the twists will end up shocking you in some way. Take for example Eren secretly being a man-made Titan in the Attack on Titan manga. Even though his "death scene" was not very convincing, the reveal of Eren suddenly transforming into the creature he hated shocked millions of readers. A nice plot twist helps to make a mediocre story possibly memorable and a great story iconic.
However, beware of cliché alley when choosing what kind of plot twist you would like to implement. Otherwise, you will end up with a completely negative reaction from the audience. One such cliché is the secret sibling reveal, which is common in a lot of fantasy novels. Personally, I have accidentally used this cliché multiple times when I first started writing. The worst example of me using this cliché was when making a story called Gold Whisker, which I made back in the fifth grade. In it, an important character known as Thorn was the main villain's brother, made even worse by the fact they obviously looked alike from the descriptions I gave in the story. (They were both black cats and the only difference they had in appearance was that Thorn had a scar on his left eye.) My experience with the cliché has made me want to warn others about its usage in stories.
Besides my own experience with the cliché, most of the time it does not add up. First off, the siblings should have known they were related in some way. If the story exists in the real world, birth certificates are available and can stop such an even from happening. Second off, the parents of the main character in most cases should have at least mentioned to them about having a sibling at least once. Finally, there are similar appearances which can pretty much give the audience a huge clue beforehand.
If you are going to implement this overused twist at all, do it in a way so the audience will not see it coming at all. A great example of this trope being done flawlessly is in the Vampire Diaries book series, where the different looking and different personality-filled characters of Stefan and Damon were revealed halfway through the novel to be brothers. Before the popular TV show came around about ten years later, this was a great twist for the audience. People became engaged in not just the romance going on, but on the brothers themselves. When done right, this cliché can work. Just don't overuse it or make it lame like me.