To Tell SIL To Stop Claiming Things?

I'd have fun with this, ask how things are going financially the next time requests leftovers. Just say that you understand that it's a tough economic climate. Say you should have realised that they were struggling financially when she's always asking for people's Christmas gifts & leftover food. All asked innocently of course.....

Prepare a couple of signs in advance (having explained the situation beforehand to guests). One says "No, you can't have any of my presents, SIL" and the other says "No, you can't take any leftovers home, SIL". Then all concerned can silently point to whichever one applies if she still tries to scrounge.

Why on earth did you invite her again? She's batshit.

She sounds nuts! You 100% have to speak to her in advance about this. Tell her to stop telling people to eat less, give her unwanted presents (as if anyone would do this whilst the gift giver is in the room!).

I strongly suspect she believes its just 'her' and her little ways and you all have to suck it up. In the meantime she gets her own way.

Of course you could say nothing and then strongly call her out in front of everyone. She clearly isnt embarassed at all.

Me - well I would speak to her in advance and do get your partner to back you up. No caving in especially if she turns on the tears (yes, I have a relative who does this when she doesnt get her own way but even she isnt THIS bad!)

Buy her a huge pack of Tupperware for Christmas. She’ll get all excited… then when it comes to hometime, tell her you’ve got nothing left over to put in them for her, but that she can use them at the next house she pops into on the scrounge.

I think I would tell her that any christmas gifts that you don't like will go to a charity shop to help the less fortunate and then go on about the charity and how much good it will do.
As for the food, I would just say no and then ask do you not get fed at home? If she persists tell her it is rude

This is really on your husband to sort out with his family. A frank word with his brother on whether everything is really ok, and he tells him to speak to his wife about her behaviour.

You need to disinvite op. When did they last invite you for a lovely meal as their treat?

The alternative is a fall out on Christmas day, spoiling your day, hard work and others' day. She's had chances and not improved.

It is already stressing you out and you will be on edge waiting for her to be her.

@schleep message is a good one if you still insist on inviting super scrounger

I have some experience from my dd1 who is a less extreme version. She arrives with her brood empty handed and hungover. Wants the leftover meat for her picnic etc. She sadly has even tried to take a rug she liked and my perfume. She just stares at me til I give in - everyone can feel the tension. She is not here this year as she has another invite...

You havent addressed whether you're actually giving a firm but friendly no to the requests for biscuits/leftovers? Sometimes when someone acts so brazenly it catches you off guard but this is a pattern, so prepare your responses. She's so out of the bounds of normality I'd suspect she actually gets a kick out of it (and I'm a very frugal person). Inform your DH so he can also give the responses and back you up.

You: Biscuit?
Her: yes and I'll have some to take home as well, thanks
You: haha, no you won't!/Haha, count yourself lucky I'm offering you one!/err no, those are my biscuits!/Haha, as if!

Her: taking things anyway when you've said no
You: SIL, what are you doing? I invited you round for lunch, not to clear out house out!

Her: no one eat too much of that, I'm going to take some home
You: err no you won't! Any leftovers are earmarked for our lunch tomorrow - but please, everyone take as much as you want!

As it's happened so many times you can address it upfront too - and by you, I mean DH.

This is the perfect way to deal with it.
What do we always say on Mumsnet? NO is a complete sentence!

When she brings up the leftovers thing I'd just say, "Oh don't worry, we've already set some aside to pack up for you to take home" and encourage the other people to help themselves, and then afterwards pretend you don't know what happened to it.

For the presents just laugh as if you think she's joking.

If I went out to dinner at someone else's I would feel incredibly uncomfortable knowing that another guest was watching everything I ate because it would mean less for her to scrounge. You really have no choice but to insist in advance that there is no repetition of this behaviour.

"No"

DH ate as much as he could when she asked people to not eat all of something she wanted so he could prove a point but again she would just push further.

That’s so funny. I have visions of a man reaching across the table grabbing handfuls Q of food and squashing it in his mouth!

Good for DH but he shouldn’t have to do that. Tell her NO we want the left overs for us seeing as it’s our food. Also laugh in her face and talk to her like a child saying “Don’t be so greedy. Buy your own food!”

When she asks for your presents just stare at her blank faces for a few seconds until it’s uncomfortable and then say no

No need for explanation, just NO in a tone that you'd use with a toddler.

yabu because you need to say No to her
its your food, not hers.

I also agree with the Peter who said that by replying that the leftovers are for the host you risk the possibility of making your guests extremely uncomfortable

If anyone said that to me I'd be very conscious of how much we were eating and wouldn't enjoy the meal.

Much better to say something like "don't be daft Maureen, everyone tuck in, it's Christmas Day!" And smile.

She is scum.
It really is that simple.
Why you would have someone near your home a second time after such rudeness?

Far better you cancel today to show them both how completely unacceptable their behaviour is.
This is who she is.
Keep her the hell away from your Christmas.

This behaviour is absolutely bizarre. It's not for you, your family or guests to tolerate it however, and I'm surprised that you have. The approach some PPs have suggested, of telling your SiL and BiL that she has two choices - act like a decent human being or stay home is good. However, following it up with a refusal to invite them next year means a 12 month gap between actions and consequences. I'd suggest instead that if she comes and doesn't abide by the rules, she's warned once. If it happens again, she's back on the road. Make it a clear, non-negotiable boundary.

This will cause a terrible scene, but a sticking-plaster torn off quickly is better than a miserable day for everybody. Once she's gone (and your BiL with her) you can all relax and enjoy the day without being on edge and wondering when SiL is going to pipe up with her next vulturous demand.

As for the future, I can't imagine wanting someone as rude and greedy as your SiL in my house again, but that of course, is up to you. Maybe a showdown over the roasties will induce a reset, maybe not. But it's apparent from the stress it's causing you that something has to change.

Just say leftovers are already spoken for sil with a big smile. Job done. No need to make a drama and potentially cause a row.

I'm normally overly polite, but I really think I would call BIL today and say if it happens this Christmas, you will ask them to leave and get their coats- immediately - and actually do it. Tell BIL you mean it. She seems so thick-skinned nothing else will work. Lots of other good advice here too; loving the tambourine idea (if you have kids they could have maracas?)

I read a way to sort this.
It is a compliment killer...
Everyone don't eat the coleslaw!
You...... oh SIL don't worry it was from Waitrose they stock it every week, you can just go buy some.
Same with any food item.
SIL , oh can I have your bag, as it would suit me.
You
Take a picture of the bag.
Tell her you have sent it too her, if she loads Google lens, she can look up where to buy it and you can be twins...
If she asks to take some food home, tell her your going to have a crap, does she want you to wrap it or flush it?

My god this would drive me insane, it’s mental behaviour! I’m not surprised you’ve been so stressed about it. I think I’d be inclined to say to my brother that if he doesn’t speak to her about it, then you will and that will be more embarrassing for her.

I'm astonished that people actually come to the dinner table with an eye on the leftovers. It's like Oliver frigging Twist.

OP has said that the BIL doesn't want her to say anything. I assume the OP hasn't.