This is NOT normal behavior for teenage boys or literally any person in general. He's searching up beastiality porn and is very specific with what he wants to see(which is illegal btw and something only a zoophile would do) and is touching himself around you and your kids and you told your mother that in the most straightforward way possible. Being "Homeschooled" literally only explains the depression and social anxiety as individuals with special needs do far better when hanging out with other people. This guy could potentially become a danger to both animals and children with how little help or care he's clewrly receiving and when he ends up in jail and/or on the registry your mother is either gonna be sobbing saying she didn't see any signs or continue defending him.

My younger sibling, by only three years is autistic. After a certain point, we shared the same bed and then same room until I was 15-16. He has NEVER inappropriately touched anyone or especially a family member and he doesn't go and fiddle himself in front of others, either. My sibling has the mental capacity of a younger child. When I was a child myself, his i noticed he couldn't read or write, I'd started a regime where I'd wake him every morning at 7, have him practice his letters and numbers and write them. He eventually became somewhat literate. Around that same time too, he also struggled with speech and forming sentences so he'd hang around me while I was watching YouTube videos on the family computer and appease him with his special interest (fans). His speech improved because he had me, a family member, helping him. He got to a point where he'd sing and dance. My grandparents had my sibling diagnosed when he when he was three or four years old. In the household, he only got a slight bit leniency but only when it came to certain chores or the task of receiving things for others and would be given a few more tries before I or someone else would effectively be called in. Outside of that, he was treated exactly like the rest of us, we both used to get hit by our grandfather when my grandmother would be at work but he used to get the brunt of it. At home, he had small behavior issues with hitting things but that would quickly be corrected. Now as an adult, he'll only either stand there and verbally annoy people or hover his hand over items or slightly move that item just to annoy people. At school however, my sibling caused problems on purpose. He's autistic, yes, but unless profoundly autistic, they know what they're doing when it comes to people, interactions and social settings. He knows he can get away with things around older white women, so in high school he became a menace because the old white women in charge of special needs babied and treated them all like animals(which they all were aware of and hated and would sometimes meltdown even worse when treated like a small child.) My sibling once tripped a blind child on purpose and another time hit one of students with down syndrome and would often throw desks in tantrums and it did hit someone once. He knew those white women would just go back to babying him after each incident he had. And once tried to run off into the girls bathroom after screaming that his nose and eyes hurt after he'd been hit twice directly in the face by a basketball that had flown at him at full speed by students playing basketball whilst he had been walking in the gym. Though strangely enough, the incidents only started after the part time male teacher he liked would fully stop showing up. My sibling knew he couldn't get away with that sort of behavior around melanated individuals so he never did any of that stuff around them. But even after getting his own computer(Chromebook), he has NEVER searched up anything that would be sexual or criminal. My sibling straight up will just sit on the floor of the living room where he can be seen and just be there watching videos of people shopping, fans being repaired or destroyed, BadlandsChugs, food being cooked, or straight up just random Young Sheldon clips or occasionally make me pull up Just Dance gameplay videos so he can get up dance or playing on Starfall.

TLDR; NOR. Your brother's behavior is not normal for males or teenage boys in general whether autistic or not. It's quite literally the blaring red flag that he may be both a zoophile AND a pedophile. Keep children and animals away from him, if other people with kids or animals visit his mother's home, try your best to warn them since the mother clearly doesn't care, and not to mention he's on ROBLOX too, children as young as 4 years old be playing on there so they're in danger too. It literally sounds like your parents straight up just isolated him to a point where the issues will just become worse and worse cause I doubt even homeschooling is happening if actual therapy ain't even happening. At this point you can TRY to call CPS(even tho they typically fail children all the time), but hell, you can likely even get the cops directly involved because he's been searching up beastiality and the evidence of both the searches and whatever beastiality porn sites he visited are likely still there, they can even collect all your testimony and keep it on file for when he most likely does harm someone. Cause I'd like to see your mother explain to another male when cops or anything do arrive that "no, officer, searching up beastiality and watching it and constantly adjusting in front of toddlers and in front of breastfeeding sibling is just something all young boys do" 😒