Can I ask - how tidy are you OP?
I only ask because to some, my house would often appear to be an absolute bombsite. To others, my house appears to be quite tidy. It depends on your starting point - if you keep a home that is pristine, then anything less tends to look messy and dirty. If my MIL saw my house on an average day then I think she'd be shocked. If my mother saw my house on an average day then she'd probably think it was quite tidy.
In some cases, there is a real need for someone to DO something about it but often, it's just that one person's standards are different to someone else's.
I'm not naturally a very tidy person, I'm quite lazy and I often think I'd rather do something that interests me than tidy up. I've posted a few times on the declutter/get organised thread and am trying to change, but the change has had to come from me, not anyone else.
Some examples:
Dirty nappies usually go in the living room bin (in bags) which is emptied at night (but sometimes the smell lingers).
There are often unwashed pots and pans in the kitchen, and if we go away for a weekend I don't always make sure they're done before we go.
My dining room table is currently covered in stuff (which is being collected at the weekend)
I can barely get into one of my spare rooms as it's full of stuff which needs to be put away after Christmas/taken to the tip this weekend/keeping for DC2
What some people see as a dirty fridge isn't actually that dirty - if she's got mould and old food and what have you then fair enough, but if it's a few splashes of stuff here and there, I wouldn't worry about it.
The only thing I'd be really concerned about would be the batteries, scissors and paracetamol lying about, but again you don't say if they were left out where a child could get them, or if they were out, but out of reach (near the top of a bookcase for example)
If someone came and cleaned my house whilst I was away, I'd be massively offended. Of course, if your sister's home is genuinely approaching something requiring the assistance of Kim and Aggie, then perhaps offending her isn't such a bad thing.