Sibling characters, something that can be a lot of fun for many RPers. Some prefer to make each sibling themselves, while others prefer to make them with someone else. Either way you go about creating your sibling characters, there is more that you have to go through to make them instead of any other character without an RPed sibling.
You have to think about how the siblings get along with not only each other, but even other family members. Is one of them extremely close with their mom, while the other is closer to their dad. If they have other siblings, which sibling/s are they each closer to. Does the oldest have more responsibilities and have authority over the younger sibling/s? Do the younger sibling/s listen to the eldest?
Understand what the family dynamic is and what the relationships are like for each sibling.
One mistake I’ve personally seen RPers make, newer RPers in particular, is having the siblings have the same exact bio, or bios that are barely any different from each other. Where in any universe do siblings ever live virtually the same life? No siblings are going to have the same life, not even conjoined twins. They may have extremely similar lives, but that’s where their personality changes their perspective. The siblings could have shared an experience, but each felt different about the experience. They could have grown up in a household with a lot of rules, one sibling felt like the rules brought order to their home life, while the other felt stuck and hated being bound to the family rules.
In their bios, it’s important to note what it was like for them having their sibling. For the oldest sibling, include how they felt about gaining a younger sibling. Were they happy or upset about it? How did the entire family feel about each of the children joining the family?
The personality is where you develop the siblings into their own individuals. You can make the sibling extremely close and similar to each other, but what makes them stand out from their sibling. Does one of them always look out for the other, or is one jealous of their sibling. How do they approach things in a similar manner, and how do they do things differently. If the siblings are mostly opposites, what makes them more alike, whether they like it or not. What qualities do they bring out in each other. Personalities are where you can in a sense, compare and contrast the siblings. Make them each into their own person.