At what point is enough enough?

@BlessThisHousehold
You are self sabotaging.

The desire to be liked and feeling that you don't deserve to be, is making you fixate on someone who you know just wants a generic in- law relationship, without anything more.

From how you describe her here she is someone who doesn't particularly want to go into depth to get to know you, or understand your difficulties.
She may simply not be a "kinder than average" person or someone who tends to think about the reasons behind certain behaviour.

She gave your children presents and asked how you were in a text, both of which were pretty standard behaviour.
She did make a point of saying that you sometimes don't get involved helping out. Understandably this upset you, I can see why, but it really is the type of comment that a person who doesn't really go into depth about mental health issues might say. It's quite a predictable comment really.

The fact that you go out your way to send messages suggesting plans puts her in an awkward position. She doesn't want to have to spell it out that she doesn't particularly want to spend time with you. You are essentially pushing her into a corner, just so you can feel the upset both at her and yourself when she pushes back.

@BlessThisHousehold social anxiety/any anxiety is not an excuse just not to do something. We all at times have to do things we'd prefer not to. And yes, I have my own MH diagnoses, but even people with excellent mental health get anxious occasionally. If you never host then no wonder SIL gets pissed off. It's expensive and a lot of effort to host, even for people who enjoy it. To not reciprocate but continue to attend is quite rude actually. I suspect you'll just say there's more to it, outing, etc etc, but I fail to see why SIL not asking how you are is so awful but it's okay for you not to reciprocate. It sounds like your husband is enabling you too.

But she hates you? Why are you embarrassing yourself? If you can't be trusted to leave her alone, delete her number

Your poor husband.
Very hard to live with someone who is determined to self sabotage and drag every one down in the process.

If your husband was my brother I would be asking him if this is really how he wants to live?

Someone who is addicted to drama/victimhood/ being upset constantly by other people ......is so exhausting to live with.

Think on. Is YOUR need for drama worth more than your marriage and your husbands regard for you?

You could well lose both if you don't cop yourself on.

Fuming at you, or at SIL?

This. I don't think the OP is that reliable a narrator.

Sounds a bit like the SIL is just going about her business and gets slagged off.

I feel for you OP, but you really need to start seriously owning why you are chasing someone who causes you such pain and upset.

Thats really fxxked up.
No one is asking you to do this.
You are blessed your husband gets it.

Yet you are determined to give her your peace.

Focus on your shit, because it is dominating you and your family.

So unfair on your husband and children.

Drop that rope completely.
Avoid her 100%.
Focus on fixing yourself, that is the only person you can ever change, certainly not her.

When I've had feelings like this - disproportionate, irrational, consuming feelings where the person I'm reacting to has no idea of the scale and hasn't actually done anything that bad - in the end I've worked out why. It's because the person is prompting feelings in you that someone else did in the past, who actually mattered.
Try to think about how SIL makes you feel,and when you have felt like that in the past. A particular time of your life, a trauma, a person (parent?).

You are unconsciously seeking out repetition of this because that's what we do (if you believe Freud), perhaps until you resolve the original cause.
SIL will have no idea she's living rent free in your head. You must feel a slave to these emotions and there's no resolution because there hasn't particularly been an incident with her for you to respond to.

Talk it over with your counsellor and keep working on it, you'll get past it 💐

I wonxer what your SIL would say. Happy to come to all the family gatherings but can't ever be bothered to go to the time, trouble and expense of hosting? She bought your kids Christmas presents but you turn your nose up at them because she bought them on Amazon ( can't you buy everything on amazon) and your dc already had what she bought. How would she know that? You snub her by asking someone else about what she is an expert in.
It is not the norm to have such a in depth close relationship with your husbands siblings, purely because they are his siblings.

But you’re the one giving her the hold over you. You are putting yourself ‘in her shadow’. It’s not in her gift to take that away.

Don't think your husband's comments are helping you in this situation OP.

But she’s entitled to behave as though she thinks she’s an OK person, and to congratulate herself on a promotion. Those are not about you. No one can make you feel inferior without your consent. No one is making you have a relationship with her — her own brother isn’t keen on her.

Once it’s a matter of her being unpleasant to you, then that is your business.

I also agree with @Ponoka7 — you don’t like her or wish her well, but you are requiring things from her. It’s classic people-pleasing.

Honestly you drop the rope and go low contact. You leave it up to DH to "organise" stuff with them etc.

It will just fade away.

This is really good advice. I hope you can take it and free yourself of this, OP.

She is irrelevant. It could have been anyone. You have latched onto her because emotionally you need people who make you feel inadequate. She just happens to fill that need but if it wasn’t her it would have been someone else who treats you poorly.

This is about you, not her. As a first step, try to recognise this: she is fulfilling some need in you. Longer term your goal is to work out why you seek out people like this. But for now you need to identify that she is not the problem.

Stop psychoanalysing her and trying to figure out what her motives are. Again this is about her power over you. You will never know for sure why she does this and you can’t control it. Stop letting her have this control.

She sees you as an easy win, putting you down to make herself look good, as you are so different.

You won't win her over or show her she's wrong. You are a chess piece.

How much contact does your DH have with her DP? She is entitled to post on her FB about the hard work it took to get her new promotion, it isn't about you. You are different people, who just happened to be related by marriage. It's difficult enough to negotiate friends and blood relationships, while working full time and with a child who has SN, she probably hasn't got the time or headspace to give you what you want from her. Is your DH's BIL checking in on him and your family? You don't really like her, you don't wish her well, leave her alone. You are being exceptionally needy towards her. People pleasers are false people and usually let someone down because they over stretch themselves. They are never real friends. As said work on yourself.