I would be careful about saying/doing anything right now since you've be threatened with a CO (cut off). Maybe after a few pleasant visits, with no talk of the state of the house, but not till then.
I might not ordinarily say that, but it seems to me that those GC need you - and for more than overnights at your house and so on. Apparently, their parents haven't just fallen behind in housecleaning but in a social life, too, for both them and the kids. Maybe some of it's cuz their house is too messy to have friends in, but that doesn't explain not going out more. Maybe it's due to PPD. Maybe it's cuz they've been focused on their wedding. Maybe it's cuz SIL has turned out to be anti-social or just a penny pincher. Who knows? But thank goodness, DD and the kids have you to help them get out and have some fun!
I can see bringing those wipes, however, and just using them on anything you happen to need to clean, as your going along. Like if you're cooking/helping to cook a meal and/or cleaning up afterwards, maybe you could whip out your wipes and just say you like using them, if asked. But I don't know - even that may be touchy.
You say a health visitor is coming soon. Have they been there before? Did they say anything about the state of the house? Do you think they will this time? DD and SIL might be more likely to listen to a professional than "good ole Mom/MIL." But maybe not.
Is there such a thing as delicious ready meals?
How do you acknowledge Easter.