To Tell SIL To Stop Claiming Things?

"No, you may not!" Is a good phrase to learn.
Especially if she says "can" assuming it will be granted, rather than "may" which does, at least, imply asking permission.
Although, seriously, does she really do this? It is so rude, not to mention manipulative.

”No, you can’t have the leftover turkey leg/Brussels sprouts/mini Magnum classic/my talc, you tight-fisted wench. Now fuck off…” Should cover it.
If she’s going to be so piss-takingly rude there’s no need for you to hold back. Alternatively just deck her.

Quick text "hi SIL, just to say thanks to you inspiring us with your frugal ways we're going to be using our leftovers for our Boxing Day meal so there won't be any to take home this year. Just thought I'd let you know so you can factor it into your shopping. Also, an amazing local charity is collecting unwanted Christmas gifts, so if you're given anything you don't want feel free to bring it along and I can donate it for you. Looking forward to seeing you both x"

but BIL has asked us to leave it alone and just ignore her because she gets embarrassed

Point out to him that you and your guests are embarrassed by this behaviour, so embarrassing her is definitely the lesser evil. You clearly need to raise this with her before she comes, and tell her that if there is any repetition you will be reminding her publicly every time that she's been told not to.

I would be really embarrassed if I was a guest at yours and sil told me to stop eating certain foods. I would be put off my dinner and be desperate to leave.

Of course you or dh should have a word with her beforehand, for the sake of other guests at least. And I mean a proper talk, none of this laughing it off or the passive aggressive comments suggested in some posts here.

This is insane!

Whenever we've approached it in the past like when we told her we didn't like that she refused to pay her share on a family meal out she basically told us she would rather we didn't invite her than expect her to fork out any money for things she's not interested in.

I'd be taking her at her word on that. Extremely unpleasant behaviour. If everybody stopped inviting her perhaps either she or your BIL would address the issue.

The thing is Op, she thinks differently to you, she sees you all getting embarrassed and pushes it so you either give it to her or you get pissed off and say no. She blatantly sees it akin to bartering for an item and either answer is like water off a ducks back to her. So if you pull her up she will either shrug it off or be annoyed she hasn’t got her own way, either reaction is of no concern but be prepared for her not to learn her lesson and carry on in the future. If you turn it into a joke rebuff then you can carry on with it in the future without it becoming a line in the sand and not ever seeing them again. So tell “Queen Mary to stop hassling the peasants for their gifts, food etc and buy her own”.

Make this the decider of when and if there are any other invites.

"God no, you insufferable twat' would be my response to every single batshit claim. On repeat. Until she got the message.

If that's too blunt, phone and tell her direct that her behaviour makes her an insufferable twat and you can't be arsed to indulge it this year.

That should do the trick!

No point doing it through BIL if he doesn't want to embarrass her you can't trust him to deal with it.

I don't think you should tell her how or how not to behave. But every time she claims or asks for something tell her exactly how you feel.

"Did you mean to be so rude?" "Did you mean to make me uncomfortable?" "Did you mean to undermine me in my own house in front of my friends and family?" "Did you mean to be so impolite and upset me?" "Did you mean to take pleasure out of me opening my presents from my beloved partner/sister/mum?" Every single time she does it call her out with a "did you mean to....?" sentence so that you draw attention to her behaviour and make her answer for herself. As soon as lunch is over make it known that it's time for everyone to go home (you can let all your other guests know that they are welcome to stay). And don't ever allow your husband to invite them again.

If she's got the front to openly say she doesn't to to pay her shares for a family meal as she's not interested, than she won't be embarrassed if she gets pulled up on her scrounging.
It's BIL who is embarrassed about his wife's behaviour.

Did she not have much growing up? It might explain her behaviour a little.

I would just be honest with her and BIL

Wrap up a really awful/embarrassing ‘gift’ to yourself. Say it’s from a work secret Santa. She can have that one 🤣

Seriously though, it’s too late to uninvite them at this stage. Just keep telling her no. What does your BIL expect you to do, seriously? Let her have everything she requests? Maybe you can completely ignore her requests, as if you can’t hear them?

I do think talking to BiL might be the key to this.
He says not to talk to her about it as she gets embarrassed. Well tell him she is not going to have any leftovers to take home is not getting any of your presents and you are sick of her scrounging ruining Christmas. If she can’t behave them if might be better if they don’t come. Because if she starts she will be told in no uncertain terms.
Someone is going to get upset but why should it be you?

Just say, I am sorry but I already have recipes planned for what is left over. Sorry, but we love our presents and will definitely use them. You don't need to be rude just keep repeating the same polite answer. She'll get the message in the end and will need to shut up for the sake of propriety. If she doesn't, you know you are dealing with someone with a problem.

@Addyview after people have sat down and before food is served, get all their attention and announce that there are no doggie bags today for anyone!!!!

‘Sandra, no. Nobody is taking anything home except their own stuff so knock it off, greedy guts.’

Treat her like a toddler.

No this is a gift for me / them.

No we are not sending leftovers.

No, this is for everyone.

BIL's out of order expecting you to ignore her behaviour. She's being breathtakingly cheeky!

Personally I think your DH should have a word with BIL and ask him to talk to her. If either of them get miffed, well, they can feel free to not attend! She can't be allowed to ruin Christmas with this bizarre behaviour. If she's like this all year round I don't even know how your BIL puts up with it!

Withdraw your offer or embarrass her. It’s good BIL says she does get embarrassed because this can be sorted. When she tells people not to eat so she can have leftovers ‘No everyone eat what you want,it’s not here for Jane to squirrel up a take home. More Steve?’ When she asks for stuff ‘that’s really quite rude Jane, I’ve only just been given that. Maybe wait a week before you try and nab it.’ Really make it uncomfortable for her, hopefully she’ll leave.

You could speak to your BIL before and tell him you won’t be accepting her usual behaviour so it’s up to him to decide if they come or not.

Not read many PPs but wanted to say she is awful. Give her copy of Dickens '''A Christmas Carol''. Did she have very deprived childhood? Anyway she's an embarrassment to family - needs counselling?

"Please stop asking to take our stuff home with you, I find it quite annoying and rude"

Say this every time.