Hi,
Unfortunately i'm not qualified in metal illness to comment other than to say that personally, i have found that its not the case as such that people that are neurodivergent wont say "i love you" or appreciate what you are doing, its more that they cant relate at that emotional level, many lack emotional maturity and just can't communicate in that same way.
Of course this is a justified emotion, its exhausting living with anyone with a condition like Autism, i genuinely feel for your plight here and its clear that your mum needs help. Autistic children like to follow a set routine, clearly he has one but it involves you tube videos to the early hours and that needs to change for sure as its impacting others around you.
I would see if you can have a sensible chat with your mum, explain your position and how its impacting you and your younger sisters. Unfortunately she is limited for options in what she can do, she is trapped in that 80/20 bubble where 80% of her attention is diverted to the 20% of the family that is the most demanding. All of you need the same level of love and attention from her but your brother is clearly sucking the life out of her.
I'm guessing external support is limited which is often the case, for what its worth you sound like a terrific student yourself, very mature and level headed for your age and supporting in a very challenging home environment, just hold in there, I'm not saying it will get better soon but, work at your studies and you may have opportunity to go to University and have more time for yourself further down the line,
Sending you strength and best wishes at this difficult time, you take care