Siblings aren't always treated the same. It's not always the best practice for parents to do the same thing for each kid because each kid has different needs. Still, it can sting a little bit for an older child to see their younger sibling getting away with things that they were heavily reprimanded for. Life isn't fair, and the family is the first place where you see that in action.
The prolonged illness of a child can often cause unequal treatment between siblings. The siblings of chronically ill kids, known colloquially as "glass children," are frequently seen right through by their parents, and although they may appear strong, they are incredibly fragile. One brother to a teenager with cancer found himself in that position, taking care of the household and younger siblings while his parents tended to his sister's needs. When her treatments ended and both of their birthdays arrived, it was clear his needs were still not being prioritized.