I can relate, oh man. My family dynamic is very similar. My older brother is a meth addict, my mom is a religious fanatic deep in denial, my dad is awesome and my best friend but I can tell they're both very tired of financially/emotionally supporting a 34 year old man.
I am 24. My bro has been doing crazy shit since I was 14 and I'd let it slide. My mom would constantly scold me and tell me "You're not forgiving enough. Jesus forgives and so should you, etc." He'd get arrested or try to kill himself and my parents would act like it wasn't happening. My brother either loves me and is talking a mile per minute (he can talk for 90 minutes about nothing) and is being creepy and keeps repeating "I love you, I taught you how to walk" or he hates me and acts like a giant child, slamming doors and screaming in my parents faces' and then taking their cars and racing them/driving drunk/generally being a fuck-up. He has no car, no job, used to be intelligent, but not anymore, I guess that's what meth does.
The last straw was for me this summer. It started in 2012 when I had my first major surgery three hours from home. When I got home, I asked my brother to please be quiet apr go in a different room and he screamed in my face. This is acceptable behavior in my parents' house, and they do not punish him, because as they say, "siblings fight." We're adults. Children fight.
I moved out when I was 20 because I couldn't fucking live there anymore, but I still need to stay with my parents when I get really sick, after surgeries, etc. and I am always incredibly uncomfortable there. My parents say that my brother tells them I am faking my illness (I've been cut open a fuckton, the disease is definitely there), that I purposefully try to manipulate what they do (but he's the one living with mommy and daddy, driving their cars, and partying with their money. They only pay my medical bills, and I am at my home 90% of the time trying to relax), it's all classic projection. He doesn't want to accept his reality so he must distort it for his comfort.
One morning this past summer, I wasn't doing well so I was staying with mom and dad. I'd had a rough night, was in a lot of pain, it was 9am and I hadn't got a wink of sleep. My brother turned the TV volume to an astronomical level. I hobbled out to the living room and asked him to please turn it down. He turns the volume up even louder and just straight up screams, "GO FUCK YOURSELF." You're so right, you don't expect people to say that shit. Every bone in my body got overwhelmed with the rage I had repressed for a decade.
Me: "You constantly go on about how much you 'love me.' So I guess you don't give a shit that I've been through hell and I've had four fucking surgeries?"
Him: "I'm not your fucking disability case worker. I don't give a fuck about you. Your problems make my dick hard."
That was it. I told him he was fucking dead to me. Nobody deserves to be bullied for any reason, especially not by their family, and especially not by a piece of shit, waste of space human who only knows how to destroy.
I talked about it some in therapy and my therapist told me the only way to do it is just to not be there or to leave when shit pops off. I haven't been to my parents' house since that happened in June. I spent Christmas with my fiancé's family and had an amazing time. It was the first year of my life that I didn't bother to do anything with my family for Christmas, (aside from going out to dinner with my dad alone) not even a visit, and I know my mom is incredibly angry at me for it, and I don't care. If/when I have a wedding it is very likely that my mother will not attend if my brother is not invited, and I don't care about that either. It's time for me to move on, so they can either walk beside me or get the hell out of my way.
I didn't cause this situation. You didn't cause yours. This behavior continues in a terrible cycle and I realized it was severely affecting my ability to move forward or even accept happiness. I struggled with it so long and I still do. It doesn't get easier. Your parents aren't going to understand because they're part of the problem. Remove yourself when you feel it is appropriate, it is not immature or unreasonable in any way, and it sends the message loud and clear. If they aren't looking out for you, you've gotta look out for yourself - don't fall for the guilt trip because the same people who lay the guilt on you continue to enable and reinforce his destructive behavior. I hope you can find a way to move away from this, figuratively and/or literally.