Refusing to eat as a means of control??

It might be an anxiety response, not eating because stress has caused a knot and loss of appetite. She may not be hungry at all.

But how does that play out in reality? It’s difficult to imagine normal life happening around this.

Given that you have mentioned the context of an argument it is much easier to envisage this being similar to a 2/3 day Stonewalling/ Sulk: everyone being on their best behaviour, walking on eggshells and time dramatically spent alone, sighing.
Have you ever had feedback about how other people experience it?

Don’t you think it would be much easier to just learn to communicate effectively?

Perhaps the op should back off of her adult siblings relationship

No, it's different when it's a mental health issue. That's why it's recognised. Or everyone with a mental health issue might be considered "abusive" when it isn't.

Sounds like a toxic relationship & OP should not get so involved in a relationship that's frankly none of her business.

It's a MH issue

When it's done to control a partners behaviour it is absolutely abusive. It's the same as dumped ex's saying they'll commit suicide if the woman breaks up with them.

That of course doesn't rule out MH issues, but it needs to be recognised as controlling and abusive.

No, it isn’t. It is just repeating back to the poster what she said, in a different way.

Think about the words- if she feels she has no control in an argument she stops eating to get control.
It is quite different to others who have said the feel nauseous/ have no appetite.

I wonder how/why OP knows. Perhaps her brother feels high levels of distress when SIL gets control via this mechanism. Because I’m going to be honest and say that “oh, but I’m not doing it to punish him” wouldn’t lessen the distaste I would feel towards someone treating my sibling like that. I would find it very distressing to be in a relationship with this dynamic, and I hope I would leave if it developed.

Lots of women have an emotional relationship with food. Some stop eating when deeply upset and some binge. It is generally not done to get at others.

We have no idea what the tone of the thread would be like if OP's brother started it. It could be "I'm really worried about my DW she hasn't eaten since we argued" or it could be "My DW is punishing me for daring to argue"

Or if OP's DSIL started the thread there are another ample handful of possible reasons.

The only way to sort this is for the couple in question to talk about it, at a good time maybe even with some sort of therapist.

I do it it regain my own control. No one else’s.

Perhaps OP is primarily concerned with her own sibling, and what he has to live with.

FFS how ignorant & rude. They're saying that when they feel out of control this causes so much overwhelming stress that they cannot eat.

Your ignorance is astounding. You need to step back if you know nothing about these issues or you risk causing further damage.

But we're only getting OP's story aren't we, & the issue might well be more complex than she's describing

I’m not sure you realize what you’ve written:

I do it whenever I feel like I have no control over an argument say or something is happening that I don’t like

isn’t this just a way of saying it is done to control other people and situations.
It may not be a punishment but it is a tactic to coerce others to not argue (STFU) A /stop doing what is making you upset.

Hopefully this thread will help you realize how horrible it is to be on the receiving end end of this.

My dh used to do this very occasionally usually at Sunday teatime especially if things hadn't gone his way.
I used to spend ages suggesting things he could have and offering to go to the shop and then end up not having anything myself because it felt awkward.
Eventually I stopped pandering to his nonsense and just said same as a pp "your choice".
Funnily enough he stopped doing it, it's manipulative arsehole behaviour.

I would think it utterly corrosive to her marriage, please god there are no children being made to look at this.

Is she a nightmare generally?

This 100%

Think a lot of people are being unfair here to SIL. you don’t know what could be going on in her head/her past etc. it’s not normal to stop eating for a few days because of life circumstances that have nothing to do with food.

Genuinely wondering if you’re my SIL because I do this. I have a history of eating disorder and it’s not refusal to eat, it’s you physically can’t. It’s a way, consciously or subconsciously, of taking back control of the situation for yourself. I can’t speak for your SIL obviously but I do it whenever I feel like I have no control over an argument say or something is happening that I don’t like. It took me years of therapy to realise this by the way and I still do it. But it’s in no way to punish my husband or anything. Unless she’s explicitly said she’s hungry don’t assume. Whenever I get like that I can easily go days without eating and I genuinely don’t feel hungry. In fact I know my mood is starting to improve when I do want to eat.

An ex-friend does it when things are not going her way. Sometimes for months, like when her child was small and life was hard. It prompted a lot of concern from people, which is what it was designed to do. Similarly when she was getting divorced (although she had been the one having the affair) - everyone worried how thin she was etc etc. It was a way of controlling how everyone reacted to her.

I would take it like a,toddler having a tantrum, don't feed the bear, or not in this case

Just leave her to it she is only affecting herself

Yes dc stopped doing it when he realised it no longer bothered me .

Yes my sil when she wasn't getting her own way it ended a lot quicker when the behaviour was ignored thankfully she's out grown that now. She mainly did it when pregnant as it bothered people more and she got a lot of attention from it now nobody cares if she eats or not she stopped doing it.

She's describing it as a third party based on the reports of the woman's husband who is her brother. I'm not saying you are not right. I am suggesting caution is required before expressing what is going on with the SIL.

Oh I don’t know, she sounds desperately sad. I often can’t face food when I’m down or depressed

A lot of posters who starts threads uses AIBU to get traffic and hopefully get an answer to their question. A lot of AIBU threads are used for traffic and not necessarily about whether they are being unreasonable or not.