Sorry in advance for such a long post. TL;DR at the bottom.
I (M38) can relate. My younger brother (M35) got special treatment when we were younger. I was always pushed into things like sports and hunting when I was younger. I was always getting yelled at for nearly everything. I started noticing at a young age that he was getting treated substantially different as we got older and when I'd point it out I'd get yelled at. One Christmas my parents bought us a bunch of gifts and after opening all of them he burst into tears and threw a temper tantrum because he didn't get a Nintendo game called Bubble Bobble. He didn't care about all the other gifts. Coincidently my mother forgot to put it under the tree. That was a trend that carried into his adulthood.
My parents helped me, I won't deny that, but nothing was ever given without exchange of some kind of work: mowing the lawn, cleaning the cars, building a fence, roofing the house, painting the house, trimming trees, and any other manual labor they could squeeze out of me. We weren't rich, very middle class. They let me drive my grandmas 1986 Chevy Cavalier in high school when she couldn't drive anymore. After high school, they gave me 1 year at college then I was cut off. I joined the Navy. When I was in the Navy, they took my car....not because they needed the car or needed the money, my dad told me "I took the car specifically so you couldn't have it." That has been the tone ever since I was a kid. My brother on the other hand was treated like the golden child. I eventually distanced myself from a terrible family relationship in my 20's but tried getting close with them again later because my girlfriend at the time was huge on family values and kept pushing me to establish a relationship.
My brother had everything handed to him. My brother got a brand new 2002 Chevy Cavalier in high school. They let him keep his brand new car through college and provided all the money for gas, insurance, maintenance, and payments. Upon graduating my parents paid for a trip for him to the Bahamas. He went to a private, out of state university. I knew it was expensive and eventually learned that tuition for said private university at the time was $35,000 a year. I learned this in conversation with my father when I asked him if he was excited that my brother was graduating after 5 and a half years he responded
"hell yeah I am, I just sent out his last tuition check for $12,000." I responded with "for the year?" He said "no, for the semester. It's the lowest tuition check he's ever had. They're usually between $15,000 and $20,000." My jaw dropped. The pink elephant in the room got gigantic.
I also learned that they had been paying for his living, paying for his extracurriculars (swim team and chorus). They also got him a MacBook Pro, but not the entry level one, he got all the upgrades and it cost over $2,500. My parents would travel to him every 3-6 months to take him shopping for clothing and groceries, perform needed maintenance on his car, take him out to fine dining restaurants, and give him money ensuring that he was ok. One time when my dad was out to do the traditional quarterly spoiling, my brother (age 25) threw a temper tantrum. My dad had a bus schedule because my brother wouldn't take him to the airport so I asked "why doesn't bro just take you?" Immediately my brother started screaming *"I'M NOT TAKING HIM TO THE FUCKING AIRPORT BECAUSE IT'S AN INCONVENIENCE TO ME AND A WASTE OF MY TIME!!"* He also threw another tantrum when I couldn't take him to the airport at the drop of a hat. I was riding the bus at the time and he called me roughly 2 hours before his flight left screaming at me *"I NEED A RIDE TO THE AIRPORT RIGHT NOW AND YOU NEED TO COME PICK ME UP AND TAKE ME THERE!!"*
I also learned that he had 2 trips to NYC with my father. They reluctantly invited me on the second trip, and it was painful standing on the street watching my dad give my brother cash so he could go and buy hot dogs from the hot dog stands.
He eventually totaled his Cavalier hitting a deer, in which case my father gave him his brand new 2011 Toyota Camry which he totaled in an accident and got a brand new Toyota Corolla.
I eventually had to estrange myself from my immediate family due to emotional and verbal abuse. I've since disconnected with my brother and I'm not exactly sure who funded his wedding or how much help he's gotten with his new house. History would show that he hasn't paid for shit.
After 3.5 years of therapy I've learned about everybody's role in the family: my dad has NPD, my mother is the enabler of NPD, I am the scapegoat, and my brother is the golden child.
All said and done, my brother acquired:
Not sure if the tantrums have ceased in his adulthood since I haven't been communicating with him.
TL;DR: my younger brother was spoiled from a very young age. It resulted in severe tantrums, entitlement, and lack of relationships into his adulthood. Adulthood categorized starting at age 30 since he has been financially supported well into his late 20's.