It can be hard to figure out what to dress sibling children as for Halloween. You could have them dress as a pair whether it's two sisters, two brothers, a brother and sister or even multiple siblings. You can get several ideas from famous duos and trios in pop culture or even from items you use daily.
Batman and Robin would be a good choice for sibling brothers. You could have one child dress up as Batman and the other dress up as a bad guy from the Batman series. This could be The Penguin, Mr. Freeze, Two-Face or even The Riddler. Another good idea for a costume would be to dress the boys up in Mario and Luigi outfits. They'll both look about the same and will be recognisable at the same time. You could also dress them up as Blues Brothers. These outfits would also be easy to put together.
Sister outfits tend to lean toward the Disney theme. If your girls want to be villains, dress them up as the Evil Stepsisters from Cinderella. You could even have each sister pick her favourite Disney princess and dress up as that person. You can choose from Belle, Cinderella, Snow White, Jasmine, Ariel and other princesses that have been in Disney movies throughout the years. You could also dress the girls up like characters from the Wizard of Oz. Have one sister be the Wicked Witch of the West and have the other be the Good Witch.
If you are dressing a boy and girl or just want something gender neutral, usually any food that goes together would work. Have the kids dress in salt and pepper shakers or as bacon and eggs. These are classic pairings and will be instantly recognisable. You could even have the kids dress up in school robes like in Harry Potter. Make the robes distinctive and give each child their own wand. You could even have the children pick which house they want to be a part of and then you could accessorise accordingly. For example, if one child wanted to be part of the Gryffindor house, purchase a gold and maroon scarf for him to wear.
Brother and Sister
There are several options for brothers and sisters. Have them dress as the famous kids from The Flintstones, Pebbles an Bam Bam. Or, if you're into Star Wars, have one dress as Princess Leia and the other as Luke Skywalker. If you want to take the Disney route, dress the kids up as Tinkerbell and Peter Pan.
If you have to dress multiple siblings, take the Harry Potter theme further and have the kids dress as the main characters of the movie: Harry, Hermione and Ron. Dominoes would also make cute costumes. Have the first born wear one dot, the second born wear two dots and so on. You could also have the kids dress up like they work in a hospital. Have one be a nurse and the others be doctors. Make sure you have one of the kids be a patient as well. If there are three kids have them dress as the three characters from Wizard of Oz: Scarecrow, Tin Man and the Cowardly Lion.