Sadly sometimes a family is split up. Maybe the mum and dad decide to live apart. Maybe the mum or the dad dies.
After a time, the mum or dad may start a new relationship.
That may lead to the child having not just a 'new' mum or dad, but perhaps a whole new family, because the new parent in the kid's life may have kids of his or her own too.
This is having a stepfamily.
The kids are not natural brothers and sisters, they are stepsiblings (sibling is a word we can use which means a brother or sister).