Emotional blackmail (crying and repeatedly telling them they have to be friends with their brother, all whilst shutting down anything they have to say about anything, pretending to listen but taking nothing on board).
Emotional neglect (It's clear she's never listened, taken on board or accepted anything they've ever said about their feelings about her favouritism. All she does is deny favouritism, defend her actions with non-apologies "sorry but we did our best, nobody's perfect" etc -but her children are expected to be perfect and to have no negative emotions about how they were treated).
Parentification (expecting the children to hold responsibility for their parents emotional state, expecting them to put their brother's wishes first all the time as if he was their child).
It's been happening all their childhood, their brother being favoured. It's toxic behaviour and harms children.
One minute he's described as "not as sharp" as them (as well as being slower and less energetic and less healthy). A phrase that has clear meaning in UK language, usually used for older people with cognitive decline. In a young person's case it insinuates he's got LD, ND, low IQ or brain damage from being starved of oxygen at birth (all the wails of but he nearly died!) or something and as a result deserves special treatment. Yet the next minute there's nothing much wrong with him, he's apparently not disabled at all, doesn't need care or special treatment, his breathing issues were fixed at 6yr old and his health described as good for the past 4 years. She wants it every which way. She's called out on something she just switches tracks - anything to make herself and DS2 into victims.
She's put her "miracle baby" on a pedestal and as a result the rest are by definition automatically lower than him, less worthy of special treatment, not equal. The attitude that if sickly son had to miss out then everyone had to, is centering him at the exclusion of the other children's feelings. Everyone else can be made miserable just so that he doesn't have to feel left out. Instead of expecting him to understand that he missed out due to his poor health and that whilst that's sad, it's nobody's fault and no reason to ignore her other children's needs. She's continuing that theme now, expecting them all to center him to the exclusion of their own feelings/wants/needs.
People are saying "ok don't be friends, that's alright, but be polite"
- it's not alright though, not in OPs eyes. She won't accept them not being friends with him, she won't accept them having their own feelings about how they were raised and her role in that, won't take responsibility for the hurt she's caused them all their lives. She won't accept them building their own independent lives with their own friend circle now, constantly trying to emotionally manipulate them into being their brother's companions.
She's trying to control them and dictate how they live. All they're trying to do is live their lives, but she won't let them. Either they do what she wants or she makes it all about herself and her feelings and what they're doing to her (which is just more parentification, they're doing nothing to her, her feelings are her responsibility). She's holding their brother up as some kind of sacrificial victim (when he's got his own interests, friend circle and hobbies/life!) and blaming them for his feelings too, when she's the one who created the feelings of resentment towards him in the first place, by blaming him as the reason why they couldn't do xyz and it was cancelled, repeatedly, all their lives.
They're too young to realise yet that everything she's done is all about her and her feelings about how her ikkle baby nearly died. Those feelings she's never properly dealt with have coloured their childhood, are still colouring their lives and she refuses to take responsibility for that.
All they can see is that their brother walks a different path to them and they don't click with him (which is enough reason not to be friends with someone) and that if he hadn't been in the family, their lives (they think) would have been different. With less drama, less inequality etc.
Personally, i suspect they're wrong about that and if sickly brother hadn't been there for the focus of their parents dramatics, their mother would have found some way to "other" one of the others and there still would have been a golden child/scapegoat/mother-as-centre-of-universe type dynamic.
So yeh, constant ongoing bullying, she's toxic as all hell and they're rebelling, standing up for themselves. She said it started a few years ago. When DS2 would have been 16 so basically an adult, DS1 would have been 17 so basically an adult, DS3 would have been 15 so basically an adult. They're all just moving into adulthood and living (or trying to!) their own lives. Three of them happen to click and want to hang out (and have been used to providing the emotional support they didn't get from parents, to each other, so are pretty close), but that's not allowed because they're not centring mummy's precious.