My SIL has a filthy house and her children are always smelly

[OprahWinfrey]

Yes, I know I might get slaughtered on here. But here I go anyway.

SIL's children are always wearing dirty clothes and smell of stale food. Her house is absolutely filthy and smells. She is very overweight and her daughter is suffering from obesity now. (11) I feel quite sorry for my brother as he likes bringing them round to visit us. On saturday I took my niece and nephew (11,12) out and SIL sent them round with clothes that stank. I knew why people moved away from them while we were out. I found out that the kids hadn't had a wash for weeks (they said!!)

The kids love it round mine and have planned to come and stay during the easter hols. I am embarrassed to take them out as their appearance is appalling. Manners are non-existent. They swear at each other and are extremely disrespectful in public. One of them smelt of pee (I am not making this up!) Can I say something, nicely, to SIL.......... or AIBU?

I think it's great if the DN's get help from their auntie for their own personal hygiene. BUT there's a much bigger picture here I think. SIL sounds like she's depressed, overwhelmed or given up. SHE needs help though she may not realise it or even want to acknowledge it.

I absolutely accept what you are saying piprabbit. Of course these girls should be responsible for their own hygiene. Given the little we know, I think it would be expecting too much for them to suddenly turn into Flylady, after what looks like long term neglect (and yes, smelling of stale wee is neglect).
I do think that there are other issues in that house, from the sounds of things, so as with anything change cannot occur in a vacuum, i.e. no good showing nieces how to wash if returning to live in a tip, unless OP is willing to take on level of responsibility which would go beyond having them for holidays.
Short fix is not going to work here, and I still maintain it is not fair to place the onus on the children to fix this, and worse, keep it fixed.
Both parents need guidance. Who is willing to give it is the real issue, as OP would feel too embarrassed (she had said), her mother upsets apple cart entirely by cleaning every time she visits SIL house, thereby alienating SIL, and brother seems to not be mentioned much at all.
What would work, longterm? (holiday wash is not longterm imo and doesn't address other issues)