Osiris1, my sister helped me take my first steps towards desqualoring. I don't have children (4 dogs and 4 cats, but no children that I know of,
) so I can't comment on them, but I'd like to share how my sister helped me:
She suggested that I watch more TV!
Our home while growing up was cluttered and disorganized, but not dirty (Mom's a bit of a "germ freak.") Dad was organized, Mom was cluttered and distractible, and Grandma was a scavenger and hoarder of potentially useful things ("If you don't need it, you can store it in the basement!") Brother was OCPD-neat (aka "anal-retentive,) I was disorganized, cluttered, and demand-resistant, and sister was the most normal of the siblings, but we were both vulnerable to Mom's guilt trips about discarding clutter ("I paid good money for this, and you turned up your nose at it!")
For my brother's 21st birthday, his only request was for a dumpster. He filled the largest dumpster our parents could rent (it was so large that when I stood in the bottom, I couldn't see over the top.) Grandma would climb in and retrieve "perfectly good" clutter, but that dumpster was completely filled when it departed.
Fast-forward 20 years. Dad, brother, and Grandma have all passed away. Mom has downsized her clutter to manageable levels. Sister is living 1000+ miles away from me. I had always accepted that I would have squalor in my life. Disorganization + difficulty discarding + pets = squalor.
Since I accepted the squalor, it never occured to me that I could improve my surroundings without considerable grieving, self-loathing, etc. I was embarrassed and often ashamed, but never thought things could change. DH and I both lived in fear that someone would see the mess, and take official action (eg, removing pets, reporting us to our professional licensing boards, etc.)
Twice, before I moved in with DH, Mom and Grandma had cleaned up my place while I was out of town. I was so angry that I screamed at them. Their hubris + my shame/embarrassment + our insight that "the apple didn't fall far from the tree" = my rage.
I "hit bottom" when a few years ago, my sister came to visit. I couldn't clean up before she arrived, because I was sick. I tried to keep her and Mom from coming inside the house, acknowledging that it was due to my shame/embarrassment over the squalor. Sister finally talked their way inside. I felt ashamed, but she and Mom were very considerate.
After sister returned home, she gently suggested that I watch "Clean Sweep." That was all she suggested, so I watched it, and experienced several epiphanies: desqualoring didn't have to be as painful as I thought! Other people had clutter, too--nice, normal people! There were skills I could learn to make the process easier! Other people didn't always know these skills, either! I had to sort everything before I could organize it! If I had too many things to keep, I could create a game to help decide what to keep! DH and I didn't have to agree about exactly how to desqualor--it was more important to me, and I'm less squeamish and quicker to act, so it was all right for me to take the initiative! If I asked myself the same questions that the hosts were asking the participants (called motivational interviewing, a la "So this item reminds you of your friend. What would it mean to you to donate it? Would that feel like rejecting your friend? Would your friend agree with this? Wouldn't she want you to live well?") I could come to understand how to improve my surroundings, while improving my attitude towards it!
There was something especially therapeutic about learning this from TV. I suspect it's because I could watch it exactly when I wanted to, pause it if I felt too anxious, replay it when DH was in the room (he's waaay more shy and avoidant than I am,) play it while I decluttered (it felt like having a supportive friend helping me,) or watch a whole string of episodes when I wanted a marathon. In addition, watching multiple episode and many different "clean your house shows" helped me identify the commonalities, instead of getting caught up in irrelevant details ("I can't organize this room until I have a tarp, and it stops raining, so I can use the front yard. Wait...I don't have much of a front yard--guess I can't do this at all.")
This paradigm shift was exactly what I needed. In order to change, you have to accept that change is possible.
I tried some of the decluttering techniques from the shows, developed some of my own, and over the next 1-2yrs, got our house more or less in order. It's not as organized as I'd like, but all the rooms we use regularily are "visitor-acceptable" nearly all the time, the pet mess is cleaned up 1-2x/day, nothing stinks, and I can have the place looking quite good with a quick "stash-and-dash." I still get choked up when I think about this journey, because before I watched those shows, I never, ever believed this change was possible.
If I had a brother whose family was living in squalor, my approach would go something like this "Dear brother, I love you very much. People who love each other as much as we do should help make each other's lives better. I'm afraid for you and your family--what if the fire department or EMS had to come to your house, saw all this mess, and reported you to DHS (Department of Human Services)? I know you love your kids, but THEY don't know that--they could ge taken away, and it's nearly impossible to 'fall off their radar' once you get their attention. What can I do to help you-all climb out of this mess? There are all sorts of things that could help: videos, books, equipment, cleaning services, support groups--what would be most helpful to you? The kids seem willing to help me--is that helpful to YOU?"
Once he tells you how best to help him, the two of you should strategize on how to approach his wife (under my principle of approaching the person with whom you have the longest/closest/best relationship first.) If possible, the 3 of you should discuss together what would be most helpful to sister-in-law.
This strategy positions you in what I believe is your best role: the understanding, loving sister, who's helping them protect their family from the scrutiny of others. This IS what you're trying to do, and is more likely to be effective than positioning yourself as an authority figure, who will involve DHS if necessary.
If you encounter a flat-out refusal, you can up the ante: "I can sure understand how you want to avoid addressing the problem/do it yourself, but that choice might not be yours to make. If you have to call the fire department, an ambulance. Or the police, they're obligated to report when kids are endangered. I want to do whatever needs to be done to avoid that situation. What can we do now, so you never have to worry about that possibility?"
Chances are, they'll say something that will help you decide what to do next. Even if you encounter complete resistance, they are then "on notice" that the authorities might be involved at some time in the near future, and what that might involve.
You could all watch "Clean Sweep" together.