Definitely DONT start tidying up, or suggesting that the way they live is not good enough etc. Surely your relationship with the people in your life is much more important than whether someone is tidy or not If you carry on in that way you may tidy your relationship and contact away too!! Be glad you are invited to visit, as you will have seen on here that many people sadly are not invited to be part of their grandchildrens or childrens lives, so in 10 years time you will not remember what was tidied or what was clean but you really will know if that has ruined your relationship with your family.
The things that might be more of help is offering to take children and babies out for a walk and give them a break. Then an offer to get a takeaway for everyone one evening, so no cooking to be done would be a treat and you will know that the food has been prepared in hygenic circumstances
Another possibility is just listen to the conversation and if you dil comments how much she dislikes ironing or whatever ou could then offer to do some for her. NOT pouncing on the toping straight away, implying her lack of whatever, but in a more casual way, so you could offer to "start" some ironing for her or whatever, and you need to respond to the way she takes it. She might be very grateful or very bristly in which case step back and dont refer to it again. Do they have a garden and do you garden? If so you can imply how you miss your garden and would enjoy doing a bit there. That should not be seen as critical of them but more you wanting to do some gardening.
If it all gets to you when you are there, arrange to go out for a walk on your own or to somewhere you would like to visit, and that could give you a break or you might like to go with them. Either way you get away from the mess for a while, but really if you concentrate on the people and not the things , I think you will have a happier and more relaxed time and keep your important relationship going.
(I would call myself an untidy but clean person!! Must have clean clothes and wash up etc etc, but my interests means that there will always be books and papers referring to two or three different interests, the local paper telling what is happening this week, sudoko, crossword and chess set, as I have a very bad back and cant keep either walking or sitting for long times, so have two or three things going at once and sit or stand when I am able. I am sure my friends would think everywhere is very scruffy but life is for living. No one will be remembering you for the quailty of your dusting when youre gone!!