I'm so glad this isn't just me, although yours seems a worse state it must be said!
My daughter is also a stay at home (she does teach a handful of yoga classes now) mum (of one perfect wee girl who even sleeps for a couple of hours during the day and goes to bed for the night around 7) although her husband works long hours and sometimes every day.
She doesn't do housework so when we go over, which isn't often, the house smells stuffy, there's invariably washing draped over all the radiators (also in summer when the heating is off!!!), piles of dirty dishes in the kitchen etc etc etc. It was much worse in the previous house which was big, draughty and damp with 2 rabbits kept in the kitchen to add to the aroma!
When they moved into this house we offered to take the wee one out a morning a week (we already do a lot of babysitting in our house) to let the daughter get on with the housework ... still awaiting a reply over a year later.
I actually want to shake her into some kind of action, but kind of blame myself because I didn't make her do anything when she was still at home.
I don't think confrontation is the way to go, and I wish I knew the answer! I feel so sorry for my Sil having to come home late at night after working umpteen hours and tidy the house.