Why should somebody get to take the credit for a vacation that doesn't affect their bank balance?
Sibling rivalries sound like something that should get left behind in childhood, but that couldn't be further from the truth a lot of the time. In fact, it is not uncommon for unfortunate behaviors and difficult family relationships to only get worse as people get older. All family dynamics are complicated in some ways, but some are more complicated than others.
Being on the receiving end of a sibling who doesn't want to behave doesn't get any easier as we get older. If anything, it is even more annoying when dealing with an adult who should know better. What is even more infuriating is that they aren't necessarily going to learn their lesson if you take action against them. It is like fighting in the backseat of Mom's car all over again.
The unfortunate truth is that a lot of people feel more entitled to take liberties with others when they happen to be related to them. They figure that because they're family, they'll have their back even when they do wrong. Of course, this couldn't be further from the truth, but this leaves the target of their conniving in an uncomfortable position. Sometimes, it causes more drama than it's worth to call them out. At the same time, if an issue of money or respect is on the line, it can feel like it is important to make sure that they get taken down a peg.