To think this is poor?

[Mailbox22]

I invited my sister round for a few hours this weekend, either Sat or Sun. She gratefully accepted. I have other plans but specifically arranged them so I could still see her (we see each other every few days).

She just rang me now and when I asked which day would be more convenient, said, oh I can’t do either now, I have other plans She does have a history of these kinds of things: starting arguments, being a bit antagonistic (I could go on) all whilst maintaining that she wants to be friends.

How to respond to this?

well as you say you see each other every few days, I would say cheery oh well, never mind see you ( insert day of week in a few days time)
Sounds like you have a busy weekend anyway

Cheery response sounds good unless there's more to this you're looking for an argument? Perhaps an opportunity to get used to not necessarily seeing each other so often. Take control rather than trying to accommodate her every time.

I'd reply something along the lines of "oh ok, it would have been useful if you'd let me know as I was arranging other plans around it, see you soon"

It was a bit rude of her, but since you both left the actual details up in the air, I would have assumed you would have pushed to fix the day/time if it was important to your other plans

Stop rearranging your plans to fit in with her if she doesn't always show for a start.

Sorry, think I misread. If you see each other frequently I wouldn't be too bothered.

To be honest if you see each other so regularly I would expect plans to change last minute on occasion.

You see each other a lot and may be she sees it as a casual arrangement.

I'd just say - no prob, but do me a favour let me know as soon as you know in future if you're going to cancel. Just helps me with my other plans that's all.

Perhaps reduce seeing each other to once a week , that may be easier.

If you see each other regularly then I wouldn’t see a problem. Especially not with a few days notice