From my experience with volunteering with people with autism there are a few main things that people don't often understand.
1) There's Asperger's / high functioning autism e.g. Can learn more quickly to adapt to functioning in society, are able to interact with others fairly well, are able to do well in school etc, but socially are a bit 'off' - as it takes them longer to learn social stuff, tend to be high anxiety and depression etc, but generally ones who will go off to uni etc.
2) Kanner's autism - low functioning, hyper or hypo sensory (either freak out and have meltdowns with any noise, light, smell, taste, touch that is too much for them as they 'over feel' everything - or hypo where they are sensory seeking - they typical thing is they flap their arms a lot, love those spinning toys, flashing lights etc) - they are below the communication barrier most of the time e.g. they have no interest in anything other than themselves, no intent to communicate to socialise, they are content in their own little world, they behave as they want when they want. They might be able to talk but only to indicate wants / or needs at the highest end of spectrum. It's a totally different thing.
3) some children who have been mismanaged by their parents and exhibit some of the less socially acceptable behaviours being classed as autistic, but if with a diagnostician, do not exhibit parts of the DSMV that would actually determine them as autistic - aka kids with no boundaries and rules that were not told no, rule the roost and parents want an 'excuse' to the behaviour - really more common than you'd expect, its scary.
Sounds like OP's brother is Kanner's and needs sensory stimulation... stimming behaviours include obsessions such as watching water swirl down the drain - eg. pouring drinks... it always is amazing to me that every kid I've met with Kanner's is obsessed with spinning - either themselves, toys in their hands, spinning lights, twirling pencils, spinning tops, any sort of repetitive motion really, roundabouts in the park etc because it stimulates the vestibular system (remember spinning in circles as a kid being fun? they don't seem to grow out of it)
a solution could be to give him a drinks bottle that looks like your one (already emptied) on the side to play with so you fill it with water and food dye to make it look the same, and he can pour away and everyone's happy.
Your mum needs to enforce better boundaries - autistic kids respond well to structure and routine e.g. visual timetables - the problem is if he's been in a habit of coming home, playing loud music, running around and yelling, this is his stimming and no one has stopped him, so in his world it's acceptable... but it your world its not.
Set up 'noisy time' so indicate to him 10 minutes of noisy time, 5 minutes left, 2 minutes left, 1 minute left - so the transition is less stressful too - some kids can't move from one activity to another well without a meltdown.
he will learn as he grows to self monitor a bit better but depending on severity, its more down to your poor mum and its hard work. she has to be super strict enforcing the rules and you need to help enforce them too. perhaps giving you a cupboard high up with your things in you could lock so your brother doesn't have access to it?
I hope it all went will with your exams.