How do I let go of this irritation and frustration with this situation?

I’m 28 and back at home after a divorce living with my mom, her husband of 3 years, and his 20 soon to be 21 year old son. Everyone is cool and we all get along great except for one issue: My little step-brother is basically, well… how to put it nicely… useless.

He and my step-dad are very close. Maybe a little too close. My step-brother has been coddled and catered to, and it’s really starting to show as he gets older. He’s never been made to get a job. The excuse is that he works with his dad, but there is no set schedule and no accountability. If he wants a day or a week off to go play, he gets it. He wasn’t made to finish high school until my mom came in the picture and made him get his GED. Now, he isn’t made or even urged to go to college. Basically, he lives the high life. All his bills are paid, he is given money, and allowed to do whatever the hell he wants with zero responsibility. He rides motocross with his dad and gotten to travel and play more than any kid his age that I’ve known. But in the mean time, he is learning no life skills or responsibility.

There is a catch to this story: He will be receiving some trust fund money at 21, and again two more times in the future. He got some at 18 and blew it within a year. It wasn’t millions or anything, but more than enough to be comfortable for a while, had he been responsible with the money.

I guess this scenario bothers me in several ways. First, he is not held accountable or responsible in any way, which irks me because I feel like at a certain point, a person needs to become a contributing member of society. Also, I feel offended if I’m called out at home for not “pulling my weight” because I feel like since I work and go to school and he does nothing, he should pull a little more weight than me. If you have no job and don’t go to school, I feel like your job is to keep the house nice, but that’s just me. Second, I worry that he will be that guy who is 35 years old and still living with his parents because he has no idea how to take care of himself. And what girl is going to want to marry a guy who can’t take care of himself and thinks life is just a huge party? Lastly, I’m afraid this is going to ruin my mom’s marriage. This is the one and only thing they argue about and there is absolutely no leeway or middle ground when it comes to this subject.

So, my question is, how do I let go of this irritation and frustration with this situation?