Fed up Guardian/Sibling?

Ok, my brother doesn't have epilepsy. But he does have ADHD and MR. My mother had some of the same problems with him(mostly the stealing), but he lies quite frequently also. He is now 18 with the mentality of a 12 year old. She was also very frustrated in how to discipline him while he was younger, and even now since he still lives with her, and isn't all the way "there". He only had a mild form of MR from being so under-developed(born at 27 weeks), but it can often times be very hard to handle and is irritating because he doesn't act his age. He lives at home, moved his gf in, is trying to get her pregnant, and plans to marry her with no job, no money, no drive, and everything he has he stole from others. She also tried talking to him about his behavior when he was younger, and it didn't work. She tried taking away privileges and time outs...didn't work. When all else failed, she would whoop his little ***! It worked for about a year and he straightened up, but after that, it was back to the same things. That no longer helped either. She has found something that does, however. I know you said you don't yell at him, but so far that's the only thing that's worked on a permanent basis for him-to get his feelings hurt. He tries to do things the right way now, has stopped stealing and lying, etc. He is afraid to disappoint because it hurts his feelings to get yelled at, or know anyone's disappointed in him...goes back to his mentality level. Children HATE to feel like they let their parents down. In fact, they had the talk about all the things I mentioned before and he got to feeling that way. Since then he has enrolled in college and gone every day, is putting off the wedding and the baby talk until they have some money saved up when he's done with school(going for masonry), put in applications, and even has a job interview Tuesday. He has left the gang he was in, doesn't fight with the siblings so much, tries to be more honest, hasn't stole anything else since he got caught with stolen bike, and shows much more appreciation and respect for my mother. Not saying that's 100% to work, but it's worth a shot. Of course, it won't happen overnight, but soon enough. I also used to be a para at a preschool and also volunteer as a teacher's assistant when I was in middle school for mentally handicapped children, and that worked for them as well. I didn't yell at them obviously. But if their actions were out of line, I did express to them what they did wrong and that I was disappointed, and they straightened up as well...just a thought. Good luck!