AIBU to think there must be something wrong?

I dunno, I’m perimenopausal with ADHD and sometimes feel like I’m bordering on sociopath territory! Kidding. But only just 😬 Contrary to MN doctrine we’re not all saints with superpowers and Christ-like empathy. We can be selfish reactive shits just like anyone else.

Anyway, to your point, OP, I get your need to understand why your SIL is like this - particularly if she still lives at home and has failed to launch, which suggests she has considerable difficulties with adult life & people & the world in general.

And it is easier to feel compassion for someone who has A Condition than it is for someone who’s just being a horrific twat all the time. There’s also something quite scary about the idea that people can just go around being nasty for no reason at all. We want to understand what underpins it - often in the hope that this might unlock some way to stop the behaviour.

But as others have said, all you can do is protect yourself and try to keep firm and fair boundaries. She sounds awful.

ADHD doesn't make people rude or unpleasant.
Even borderline personality disorder doesn't necessarily do that.
And menopause isn't an illness or disorder?
You sound quite ignorant.
What difference does it make what you think as she's hardly going to go and get a diagnosis just because you think she's horrible?
You don't seem like you want to help her either.

It does sound like she has some behavioural and emotional problems , Tell her her behaviour isn’t normal.

It's a complete head scratcher then OP.
If your FiL is complicit in her behaviour there's not much you can do. Must be horribly frustrating for your DH.
Maybe your hunch is right & there is something going on with her you're unaware of. Or it's just your FiL is incapable of standing up to her.

I agree! My son is Autistic and has ADHD and is the most politest kid, he is so respectful so using ADHD as an excuse for brat behaviour isnt right.

I think it’s pretty understandable that you’re seeking answers and wondering if there’s something going on that would kind of explain her particular combination of odd life choices and hurtful behaviour.

It’s one thing to challenge her on it and hold her accountable if she’s just a nasty person — quite another to try and do so if she turns out to have a mental illness or something.

This. She's always been a nasty piece of work, she's getting worse because it sounds like there's never been any consequences for her of that. She's still living at home but makes her mum cry most days? Honestly, sod that. If that was my sister treating my mum like that i'd have her bags packed and out the door.

She lives with her parents, no bf/family/children so no family stresses. I feel like telling her perhaps she should see a Dr as her behaviour is getting worse and worse.

See, the thought of being old enough to be going through menopause and living with my parents does sound stressful to me.

Unless there's a specific reason (disabilitiy or caring duties, for example), it's unusual and smacks of failure to launch. That hints at some sort of issue, IMO. Perhaps it's frustration with her life or general unhappiness?

Whatever tbe reason, it's not really your business unless she asks for advice. I'd be staying away as much as possible.

Is she manipulative? Or able to charm to get what she wants? If so then she is aware of her bad behaviour.

Stop being such a load of wet blankets and stand up to her for goodness sake. Why is her brother allowing her to be so vile towards their parents? Why hasn’t he read her the riot act? Undiagnosed whatever or not, it does not give anyone the right to behave like a disgusting piece of shit towards others.

I don’t understand why you’re set on pathologising her behaviour. What difference does it make to your response whether she pushed your son because of some possible undiagnosed condition which makes her aggressive or because she’s a deeply unpleasant, violent individual?

If you feel unsafe around this person then it's time to distance or speak up. The MIL needs to stand up to this woman quite frankly if she's living with her what a nightmare.

Early onset dementia could explain a change in behaviour, personality and coordination like you say about barging, it can cause mood swings.

Personality disorders can present these kinds of behaviour but it's really a mute point as she's a grown adult and has an understanding her actions have an impact on other people.

These people need to be stood up to, otherwise they think they can continue to bully and trample over everyone else. I know easier said than done but probably a good thing she doesn't have family of her own to pass down the toxicity.

How did you deal with her pushing your son? You didn't say how old he is but I'd find it really difficult to ignore that sort of thing. However young/old my kid is, you assault him in front of me at your peril - I would not be able to hold back from lambasting her for that. I know it wouldn't hurt a person but it's the total lack of respect...

Are people afraid to confront her because she is likely to become violent, or is she just abusive in her speech?

Enabling these people is what makes it worse; whether there’s an underlying reason or not, it’s that they get to act like this without consequence.

I've come to realise as I get older that some people are just plain nasty and self centred. On mn it's also referred to as narcissism, it's not, it's just being completely focused on yourself because you are jealous of others and so make sure you are the centre of the world or being so selfish that you don't care about your impact on the world around you.
Thankfully, as I've got older I will tell these people exactly what I think of them or their actions...an example this weekend, the parent who stopped their family in front of a flight of steps, blocking any one using them...my response was a loud "no you carry on I didn't want to walk up the steps anyway" embarrassed them into moving.
I've also cut out anyone who behaves in such a manner from my life, except my sister in law who I am/have put up with for mil sake...but when mil passes, she will be in for a shock because she is so.self centred, rude, and unpleasant it's likely the whole family will cut her off.

Can you embolden MIL to ask her to move out?

stage an intervention

What on earth does her being a rude a-hole have to do with ADHD?!