16yo undiagnosed brother’s behaviour and habits are really affecting family/household. Any advice appreciated

My (26F) little brother has been difficult since he was a toddler, with his worst behaviour being when he was between ages 7-12. I’ve always suspected he’s on the spectrum and since asked my mum to get him diagnosed, and she never has (she’s a proud person who doesn’t believe in spectrums/adhd etc. She comes from a generation where you’d suck it up and deal with it).

Amongst various behavioural traits that don’t seem developmentally normal for his age, there’s this habit he’s had for almost 10 years now that I feel has been very detrimental to his mental health as well as my family’s. I moved home around 18 but recently due to financial reasons have had to move back to the family home. And now I am faced with the reality and extent of his issues and how my family deals with it.

The habit I’m speaking of is a very compulsive one, where he basically spends all day in his room unless he’s at college and then around a certain time, usually the same time in the evening, he will go to the living room, switch a fan on, put the TV to a specific channel (Food Network) and will just cook his dinner and do whatever he does down there. It’s his ‘alone time’ as my mum puts it. I do really feel sorry for him and can understand that he’s a teenager and he lives in a house either women (my mum, grandma - and me again now), so this is his allocated slot to do what he wants. However, what I find very bizarre and unfair, is how he demands us all to go upstairs to our rooms when it’s his time to go down. And it doesn’t matter if we’re in the middle of eating dinner, or watching the tv, he will either murmur or signal to go upstairs. He gets very impatient, especially when our curfew becomes apparent and his time to go downstairs approaches.

My mum and grandma have completely accommodated this behaviour and see it as ‘normal’. If I challenge them or question them about it, they either get defensive or act like nothing’s wrong and that I need to adapt to my brother’s quirky behaviour. They’ve been completely controlled by a 16 year old, in their own house. And it pains me for my grandma, who has bad legs, to have to be told to go upstairs by her grandson and be anxious not knowing when she can go back down or not.

If anyone’s had similar situations or can offer some advice, I would be so very grateful. I just want my mum (most of all) to take responsibility and see that this behaviour is not acceptable, even if he is most possibly on the spectrum/other issues.

Thank you :)