To Tell SIL To Stop Claiming Things?

In a way, the fact that her behaviour is so completely over the top and batshit is to your advantage. There is no one on earth - including your other guests - who would think you weren't justified in responding very firmly to her if this nonsense arises this year. The trickier things to deal with are where someone is a 'bit' odd or mean or quirky. She is so far off the reservation, you will be able to bring out your big guns without you looking bad.
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Then you should give her that choice. Give her a call and ask her to stop doing this, and that if she feels she can’t and would rather not come then you fully understand

Bil doesn't want to embarrass her, she needs to be embarrassed it sounds like its the only thing that will stop her, very odd behavior ....

What a stupid idea

As PPs have said I think it's something that would be better tackled "in the moment," with a bit of a jokey angle. If she asks for a present you've just opened, "Hang on a sec Jane, give me a chance to enjoy it before planning a takeover!"

If she starts going on about taking home leftovers, "Uhoh, did you forget to order your Christmas shopping in time? Don't worry, I'm sure we can make you a sandwich to keep you going tomorrow until the shops reopen. I'll send you a calendar invite for next year! Come one everyone, I'll take offense if there are any leftovers from this meal!"

I would NOT be using a gentle tone I’d be using a tone of voice and sentence that is serious enough to cause an awkward silence
create that awkward moment and keep creating them ( politely and non aggressively) and her husband and her will eventually have words
do a Paddington bear stare and say “ please don’t say things like that it’s embarrassing then stare and pause “ it’s the pause that will make it awkward
Stare “ are you seriously asking me to say I don’t like this and give it to you ? “ ….pause …how do you think that would make ( giver of present ) feel? ….pause… if i didn’t like it I’d get a refund for me … wouldn’t I ? …Pause …. Stare
try to breath slowly in the awkward atmosphere
Be prepared for non of it to work …

More good advice, @pimplebum
And by agreeing with the "gentle tone" I didn't mean to dodge handling this, just to deliver the message calmly and quietly, which can be a lot more effective than outright rudeness and avoid claims they're being "attacked"

I'm a big fan of letting the people who are making something awkward or embarrassing feel awkward or embarassed or ashamed themselves. We're very nice as a society and try to prevent people from feeling like that as a general rule, but that system doesn't work when somebody whose behaviour should make them feel that way is shameless about it. Allow her to feel the slightly unpleasant emotion, don't carry it for her, that's what she's relying on in order to continue with her outrageous behaviour - and her H is part of the problem!

I'd rip the plaster off and ring DBIL now and in right NOW and avoid procrastinating dealing with it head on for any longer.

"DBIL, if DSIL behaves in the way she has done in previous years it will put a shitter on Christmas and it's not something I'm willing to tolerate any more. Either you tell her in advance that she must refrain entirely from asking for leftovers, other people's gifts and generally acting like a deprived character out of a Dickensian book, or don't bring her. If she displays anything like that behaviour I will be telling her firmly and not kindly to stop, and if she does it again you will both need to leave immediately. It is not acceptable behaviour in my household or indeed in my company, and everybody else should not have their Christmas shat upon from a great height because she will feel a bit embarrassed to be told to stop behaving inappropriately."

I'd be billing them for anything they try to take. That might stop BIL telling you to let her do what she likes and not make a scene. I would also wonder if she has any childhood trauma with lack of resources, or obsessive compulsive conditions, as thisnis very far from socially acceptable.

It definitely needs sorting and the suggestion above is indeed very good.

Hopefully Freddie Freeloader won't come.

I wouldn't invite her tbh. She sounds selfish and thinks the world owes her but scrounging. People like this makes me really angry.

I would NOT be using a gentle tone I’d be using a tone of voice and sentence that is serious enough to cause an awkward silence
create that awkward moment and keep creating them ( politely and non aggressively) and her husband and her will eventually have words
do a Paddington bear stare and say “ please don’t say things like that it’s embarrassing then stare and pause “ it’s the pause that will make it awkward
Stare “ are you seriously asking me to say I don’t like this and give it to you ? “ ….pause …how do you think that would make ( giver of present ) feel? ….pause… if i didn’t like it I’d get a refund for me … wouldn’t I ? …Pause …. Stare

try to breath slowly in the awkward atmosphere

Be prepared for non of it to work …

i saw a documentary once about people who were pathologically tight like this and they needed a top notch therapist to help them and at the end they were still bad , it’s like hoarding it has deep roots and very hard to treat

next time you have a meal out make it very clear repeatedly to her “ this meal will cost you £60-80 roughly and you will be expected to pay don’t. Don’t come if you are going to fuss “

She sounds like she just doesn't care enough about seeing family. Does she ever host?

@Addyview She sounds deranged.
What a bloody cheek...She's a natural born scrounger on a good wage..

But did she know extreme poverty in childhood?
{unlikely} That's the only reason I can think of to explain away such grabby vulture like {as another PP said} behaviours.

I'd uninvite her.

Awful behaviour.

Absolutely perfect ... in other words put the embarrassment back on her and explain how she's making others feel

She may well not care and you might even get tears, but if she storms off I'd say that's a win, and providing you stay calm when telling her you'll be in the clear

Hmm, doesn't sound like she's particularly embarrassed...!

Christmas Day is supposed to be enjoyable. Your guests must feel so incredibly awkward being “monitored “ about how much they eat! Fuck that. I would uninvite her and explain why!

You and your guests can then enjoy Christmas Day dinner in a relaxing environment.

You beat me to it! I wax going to suggest exactly this.

Someone upthread suggested a copy of a book on etiquette. How about Delia's Christmas cookbook too as she'll probably be cooking herself next year

My god, she sounds very unwell. I think she herself would benefit from people telling her she can't have stuff for free, she needs to get back into the real world!

Every time she asks for something, say NO with a face that says "are you fucking mad?" Or even said this out loud to her.

Alternatively, laugh at her and say something like "here we go again, the Leftover Freebie Queen is doing her thing". Followed by "No, you fucking cant have anything, fuck off."

For Christmas 🎄, gift this woman a recipe book with all your own recipes written in it if she likes them that much she can bloody cook them herself .
And as for the asking at the actual meal for the leftovers , say no we are saving that for lunches .
As for trying to take your own Christmas gifts say no I will use that ,perhaps you ought to put that on your Christmas wish list for 2025 .
She sounds awful

Invite them to come as always.
then when she asks for your recently unwrapped gifts and asks for it reply with something such as “how rude to ask me to give you my gift that someone has thoughtfully chosen and given to me.”

Ditto when asking for food, reply with, “shall we just enjoy the meal now and if you don’t have enough food at home let us sort it out privately!” Or “rude again!”

Fuck that for a game of soldiers. Just tell the cheeky mare “No, you can’t” and don’t invite her again.

I'm finding the PPs suggesting OP just say no in the moment or give a chippy response really annoying. I'm sure OP has tried all of the above if she's lying awake at 5am and wanting to rescind the invitation.

The SIL is obviously beyond normal societal responses or hints.

It annoys me in the same way people would suggest I just say no to my (then hoarder) DH bringing stuff into the house.

People like this have developed a huge resistance to polite hints, funny remarks or even a direct no.

I also don't agree with the suggestions not to invite them again and be content with that. Why should OP have her Christmas ruined?

I say phone BIL and tell him his choices are to issue strict instructions or don't come.

I think they both need to be told that it's not on. Her husband is enabling her behaviour. Do this in front of everyone.