Hey there kiddo! So, you know how sometimes when people get married, they already have kids from their previous relationships? Well, those kids become something called "stepsiblings" to the new kids that come into the family.
Think of it like this: you have a best friend at school, right? And sometimes when you go to their house, you might meet their brother or sister. That person would be your friend's sibling, right? Well, if your mom or dad got married to someone who had a kid already, that kid would become your "stepsibling", kind of like a sibling that you got through marriage!
Now, it might take some time to get to know your new stepsibling and get used to having them around all the time. But the important thing is to be kind and respectful to each other, just like you would with any other family member. And who knows, your new stepsibling might even become your new best friend!