In-laws take our belongings

I'd only let them in the sitting room and clear it of anything liftable before they arrived. I also wouldn't leave them alone for a single moment.

I wonder if your SIL grew up in a house where she didn't have many nice things and resented people who did.

Whatever, it's theft. You could buy her a throw for her birthday or Christmas, though!

@MaidOfSteel the Queen Mary comment made me laugh out loud! 🤣

I wouldn't allow a thief in my house. From now on you visit them every time.

Just to check, these are bought throws rather than hand-knitted blankets made by her (and your DH's) beloved deceased granny? So there is no reason why she might believe that you are the ones stealing them and she is the one taking them back?

If your dp doesn't see what they're doing as stealing, and won't stand up to them, then he needs professional help as he's been conditioned to enable their abusive behaviour.

Until he is able to support you and stop them stealing from you, or until they can stop stealing from you, do not have them in your home.

They would no longer be welcome at my house. If you want to see them,then do that somewhere neutral, take the bare minimum with you. If they ask why just say I've collected my same 2 throws form SILs house 3 times, I don't want to have to fight with you about it, why do you think it's ok to steal from us.
Edited to say - just don't lend anything again, and tell them why

I think you need to learn (or fake) a really thick skin. I would turn down requests to borrow things, but if you feel like you have to let them, go and get it back as soon as they've used it. Don't be apologetic at all, just confidently say you are returning it to your house.

Watch them at your house and stop them at the door if they try and leave with something you haven't agreed to them borrowing. And if you find they have taken something without asking, make a calm fuss about it regardless of how awkward it feels, preferably with an audience. (SIL, this is mine. I didn't say you could take it? DH, did you say your sister could borrow this? I'm so confused at why it's at your house? Did you just help yourself to my throw/picture frame/hamster?). Don't shout or be aggressive, just flummoxed as to how they could do something so unreasonable.

If you want to keep a relationship without banning them from your home, you need to make it too awkward and embarrassing for them to steal your stuff.

Your SIL is a thief and your MIL is enabling it.

They can no longer come in your home. They're thieves and you don't trust them.

Stop lending them anything.

Have nothing to do with them. This thieving has been permitted and enabled so long it's not going to stop.

Stop being nice and wishy washy with phrases like do you mind if I take my stuff back? You need to call the stealing what it is. Name it.

When you allow known thieves in, You're enabling them to steal from you.

Can you give examples of what defensive things they say when you ask for your stuff back?
Youre after advice and I’m afraid the only advice in a situation like this is to be firm and just ask for your things back, and even question why they have them. “SIL, why are my throws on your bed? I’m taking them back now.”

This. There’s no way I’d have them over the threshold, the cheeky fucks.

I wouldn't have them in my house and if they asked why I would say because you steal our things.

I would not be inviting them over

and if I do I would be checking bags in the way out

This is something some girls do as children and young teens but grow out of. To continue in to adulthood makes it a major personality and judgment problem.

This is nuts, I wouldn't have them in my house. There have been a few similar threads recently about in laws stealing stuff. So weird.

yes honestly it's odd you kindly ask them if they mind you take your stuff back that they have stolen. Declare loudly: my X! How did this get here??'

Take their coats and bags as they enter. Engage them in conversation whilst doing so so that they can't refuse. If they do you cite that you need to put stuff out of the way or the kids reach and there isn't enough room in the living room. Be ready and waiting to respond as cf often do so well by catching you on the back foot. I would keep your bedroom door locked and make sure there are minimal things on display in the living room. Minimise the ability for them to wander around your home. We for example have a downstairs loo so I direct guests there.

If they do manage to nab something when you see it in their home pick it up and take it back home. Don't ask for it back, just take it. They are relying on your sense of decency and manners. Be brazen,these are your things and noone has the right to come into your home and take your hard earned belongings.

Tbh I'd just ban them from coming over but you don't sound like that is a step you are ready to take.

Hide the throws in your wardrobe next time she comes is another option.

No more lending & preferably no more visits from them.

"No, I don't want you in the house because you steal"

“These are my throws, please stop taking them or else you won’t be welcome to visit anymore.” “No you cannot borrow x, y and z as we struggle getting things back from you. You can buy xxxx at xxx store.”

Personally i would stop inviting them over.

I'd refuse to have them in my house! And be careful when invited to theirs.

“my SIL takes the same two throws which end up back at their house being used in her bedroom”

OP, I’d publicly say to her, “These are my throws and they stay at my house. If you take them again, that will be the end of any visit by you to our house. Ever.” Your MIL and SIL are brazen thieves - you need to be brazen back (and mean what you say).

You need to be direct.
You want to borrow the jug? Ok. What day will you bring it back? Don’t forget!
Oh, that’s my throw. I’m taking it back home.

Use firm, unemotional language. Don’t feel embarrassed or express anger. Keep it very neutral.

And put the things that are clearly very tempting out of sight when they visit. It’s clearly a compulsion of some kind, so try and preempt it.

This is stupid ,why have thieves in your house ?

A HAMSTER?!