I just don’t want to do secret Santa!

Just say ‘we don’t like SS’.

How about everyone lists a couple of things they'd like, with no obligation to stick to the lists? Then people who enjoy choosing presents can carry on, and SIL and anyone else who finds it stressful doesn't have to guess.

(I'm also the Scrooge who hates presents, I've yet to find a way to bow out that wouldn't spoil things for anyone else, but that would make things better if my family would only cooperate😅)

I can see it both ways to be honest. In DH's family we did it for quite a few years as there were 7 adults and 6 teen/young adult offspring and it made sense, especially since we all lived a long way apart, with some overseas. Saved rather a lot on postage too. It has all petered out now, as now there's another generation of babies & pre-schoolers, so we just buy for the kids.

From my point of view though, it did mean that I got very few presents as I'm an only child, both parents long gone and no other family to speak of.

I do buy for some people, and enjoy giving, and don't really expect anything in return.

What I didn't like was the 'I'd like [expensive gift]' that was reciprocated with some back-of-the-drawer regifted but inappropriate items. Those people were crossed off.

Only Mumsnet could accuse someone of being selfish because they WANT to buy gifts for people

OP can still do that (and her husband and whoever else) without her poor attitude towards her SIL though. Look at OPs comments,

she either will not get the message that we aren’t interested or simply doesn’t care and wants to push what she wants anyway

I could not give less of a shit if she and BIL don’t want to buy us Christmas presents because of money or laziness or whatever else

I think it’s more cba to think of presents than money tbh!

Maybe the SIL is tired of buying stuff for the sake of it.

OP is an adult and can buy what she wants. SIL is an adult and can dip out of it if she wants. It would all be more pleasant without the passive aggressive comments though.

I'd hazard a guess that she doesn't want whatever tat you all keep giving her. It's getting presents not giving them that she's objecting too. Most adults have way too much stuff. Why would you insist on adding to it just because buying and wrapping gifts makes you feel good.

This doesn’t work for most people. I couldn’t not buy someone a present knowing they were getting me something.

A compromise would be you agree not to buy them presents and you don’t buy them one, if you both have kids you do kids only etc

So tell her and her husband to not buy anything.

You (and your husband and his parents (is that the right relationships?)) buy her and her husband presents, and they can give them away if they don’t like them, like you said upthread, ”Then she’s free to give them away isn’t she”.

Then everyone is happy, especially you as you’ve still had the opportunity to buy things, you said ”I enjoy choosing presents and wrapping them up nicely for people” and you can still do this.

I think this sounds like the perfect solution, what do you think?

just say 'SIL, you've asked about SS for three years in a row now, and I think we've all been pretty consistent about not wanting to do it. If you want to stop gifts between the siblings/adults or keep to a set amount, happy to discuss that, but I don't see the point in asking about SS every year when we've made it pretty clear nobody is up for it."

Each time you've said no, what has she said? You'd think she'd get the message by now especially if the wider family also don't want to do it

Good idea, just agree not to give or receive presents from that couple.

She’s not really free not to get you all a gift is she - it would feel really one sided. I’m absolutely team secret Santa I’m afraid, due to financial constraints. We’re always hard up. It’s not laziness

I don’t want to do secret Santa , so will stop buying you gifts if that makes your life better

She doesn’t want your carefully chosen gift.

Some people will not want to be bought a gift and not reciprocate. She would probably feel bad not buying you a gift if you were to buy her one so now she feels she has to buy for you.

Is her husband your PILs son? Where is he in all this? Does he leave all the present buying to her? Is that the issue do you think?

I’m team ‘buy your own shite’ and team ‘let’s set aside some time and have a fabulous meal and game of Monopoly (cheating allowed)’.

Team ‘no pressure’.

SiL and I have always enjoyed bargain hunting and years ago , when we were skint with young children, set a very low budget limit for adult presents. It was enough for a few books from a charity shop, or something pretty from a boot sale. It worked for us, we knew each other's tastes well enough, and meant we could pick something appropriate, and do the Christmassy thing of having something to wrap. We started extending it to the DC, they would get a pre-agreed amount of cash plus something inexpensive to open, maybe vintage comics for older ones or poseable figures or small bags of lego for younger ones. For pre-school children , it's really easy to find secondhand bargains in good condition, and they don't know or care .
So if it's possible that money is the issue, you could suggest something like that. If the issue is not money, then suggest she can get everyone giftcards and you can carry on choosing for people as you see fit.