Yeah, people who do it as a joke are just assholes. But in my opinion if there genuine and just trying to help I say it’s for the better. Saves us both time. Your little brother sounds like he deserves to live with a stutter for a day, to simply see that speaking itself can be a struggle. People just don’t understand. Hope your mom told him off. If not, I think you should tell your brother that it’s not nice and hurts you. If he’s not truly a demon, he may understand and stop.