Brother decides last-minute to join sibling's PhD graduation celebration abroad, complains when he has to pay for accommodation: 'He took my spot in the already-booked Airbnb'

Why is it so hard for siblings to be fair? 

A brother decided at the last minute that he would fly abroad to celebrate his sibling's PhD graduation celebration; however, because he was so late in deciding, everything was already booked. Instead of figuring out his own accommodation and flights, he stole his brother's spot in the rental house, then refused to pay his fair share. So at the end of the day, this sibling not only took over the travel plans but tainted the entire celebration weekend with his entitlement. 

I have an older sister, and we've spent our lives at odds about fairness. We're the type of siblings who slice a cake and give the other one the smaller slice, and we're the ones who taste both coffees and decide to keep the one we like best (regardless of whether or not it's the one we ordered). Although it's not always pretty, it's always in good spirits, and this sibling rivalry is executed in a fun way. It's lighthearted; we don't actually want the other to suffer. 

Yet, in this story, it seems like the brother just wanted to take a vacation for free, regardless of whom it cost to pay for it. In this case, it cost his sibling, the one whom the whole family was flying across the world to celebrate. He couldn't, for one weekend, let his brother be the center of attention without gaining something for himself in the process. 

Although nothing is every 100% fair when it comes to sibling relationships, it's certainly not fair for this guy to come in at the last minute and ruin his brother's PhD celebration, stealing his spot in the accommodation rental, and claiming he deserved it. The family was at odds over who was right in this situation, but it sounds to me like the sibling getting his doctorate should be the one given a free place to stay and a paid-for flight. Mom just wanted to silence the waters of strife, but there are some sibling feuds that need to rock the boat a little in order to get solved fairly.