To tell my sister she can't stay over

cue tantrums and tears from her and gravely concerned/hurt calls from our parents

Tough shit, truth hurts, I wouldn't be sugar coating anything as an adult who is self aware and unwilling to put up with or enable other peoples bullshit, especially those who trample boundaries and are horrible to me and my family.

Sounds like your parents created a monster and everyone has enabled it all her life. It's stops now.

You could say - you want to come and stay but I don't want you to come and stay because you are demanding and horrid and you expect to be waited on, boss my children around, never help or contribute to anything, you then act put upon if any of this is mentioned, you've broken our relationship by a thousand cuts and i wont host you. Have you thought of AirBNB?

I’d say you’re not having guests to stay right now but you’re happy to meet her for lunch at a restaurant.

Don’t even suggest an AirBnB. Offer lunch in a restaurant and that’s it.

Hi Sister, We’d love to see you for lunch or dinner a couple of times during your visit but I’m afraid we are very busy and we can’t host this time. Here’s a list of Air BnB’s nearby.

WTF - why can't you just say No I won't allow you in my home because you're so horrible!

Tell the truth and to hell with it

Anyone who turns up and tries to boss my children around wouldn't make it through the door again. Set them a good example and show them that you have sensible boundaries. Obviously your parents/sister know if they make enough fuss, you'll give in. That's not a good life lesson for your children.

Do your parents live close to you?

No, I can't host you.

Maybe next life.

"Hi Jane, that won't work this time - have you looked at Air BnB?"

"No, the kids need their sleep for school so we aren't having visitors. Have you asked mum and dad if you can stay there?"

"Hmm, if not a great time for us but we'd love to pop in to see you at the parents place"

Or whatever works for the situation - easier if you would have to shift kids out of rooms to accommodate a visitor.

Sometimes you just have to pull the pin....

So it's your feelings or theirs?
Put yourself first op.
They care not a jot about your wellbeing.. Feel no guilt..

Oh my god, do we have the same sister?! Just this morning I was thinking whenever I help her, she totally abuses my kindness and just takes and takes from me and just wants more and if I dont help further, Im horrible. She is on the spectrum and just lives up the road and always knocks round (no boundaries). I dont know what to do because she is my sister but she gives me huge anxiety! It's like she feels I 'owe her' just for being her younger sister. It is a really challenging situation.