Parents don't clean and I struggle to be assertive

I (24F) moved out of my family home six years ago, when it was a bit messy but nothing major. Fast forward six years and there's stuff everywhere on the floor and on the sofas, damp on some of the walls, and every time I go home I ask if I can help to tidy and my mum says no because she will do it, but next time I go it's never done and is even worse. My mum and step dad live there with my brothers who are 21 and 10.They all do the minimum (wash pots, throw rubbish away etc) but there's just clothes, toys, things EVERYWHERE, and I think it's a combination of it getting on top of them and being overwhelming, but also a bit of them being oblivious because they see it every day, I see it once a month. My mum showed me my brothers room the other day and pointed out how messy it is, when there is mess everywhere in her room. Like to the point where you have to hop across the carpet to find a place to put your foot on.I wouldn't be as bothered but for my younger brothers sake I want him to grow up in a tidy home. My step dad also has asthma and chest problems and I can't for the life of me understand why he wants to live in a dusty, damp home. I want to go there for one day to do a massive clean but my problem is my mum is very sensitive and has fallen out with a LOT of family members and I've struggled to be assertive all of my life for fear of upsetting. I wouldn't like it if someone came into my home and told me I need to clean, but I know I need to say something along those lines. How would you tackle it? Don't hold back!​ via /r/Parenting http://bit.ly/2BMfJvp