Your husband is 100% the problem for never addressing their inappropriate behavior; I have the same type of man but thankfully my steps do not live with us and I have nothing to do with them because they have been molded into rude, insulting, crude and selfish women. I do not blame them, I blame him.

If they lived here, I doubt I could live with him; he has no backbone when they are around and turns into a scared weakling who nauseates me.

I wish you the best if you are dealing with the same type of man. The only hope you have is staying away from them and in your situation, I am not certain it is possible.

For my 2.5 cents, I would be shocked when someone just "takes" my stuff, especially since these are adults (oldest 2) and the SD17 is almost adult - this is old enough to know better.

Since you have no other options presented you must stand up for yourself and get a little crazy. You've already told them, and you have not given repercussions so there really is no reason to listen to you - they know you don't mean what you say. You ask about repercussions. What repercussions are you referring to?

Basically lock down your things. SD is not allowed to use your bathroom, or better, put your personal items in a bag/box you keep in your closet. That should give hints. Lock up those keys, to the spare, or hide them, definitely don't give out the keys to your car. Take out the battery when you leave. Or battery cables.

Your bedroom is OFF LIMITS. If you must put locks on the bedroom, do it. Then you can relax. See what happens. If you are that frustrated, extreme measures should be taken.

When are those kids going to launch? They need to be out on their own, if they cannot respect you.

It is unfortunate that your DH does not require them to respect you, that is on him.

I am fortunate that SD17 and SD10 pretty much respect my things and don't take anything not theirs. They ask and sometimes I say "no" and that's it.

"like the SD19 locked herself in the bathroom for over an hour doing her hair - with MY hair products and without asking first if she could use them."

Tell, don't ask, Dad to set up a 'beauty' vanity spot in her bedroom. He needs to include whatever products his daughter uses. Your products are yours, not community property. One bathroom in the home? For six people?

"Or SS22 taking my (spare) vehicle without asking because he doesn't want to waste his gas or put miles on his car."

Why this this adult (who is already living on borrowed time in the home) have access to your car keys? Lock up keys and put a stop to it. Are you seriously going to sit by and allow this just because Dad doesn't care? Find your voice. Use it.

"Or SD17 going into our bedroom to retrieve her dad's very expensive camera so she can go take pictures of snow on the trees."

Install a lock on your bedroom door. Don't ask, just do.

"They don't ask, they just take. Their father will not reprimand them for this - they usually do not even get told NOT to do it. He just doesn't see any of this as a problem worth addressing - and I have tried to help him see this, but he just doesn't see it. So there is no help there. "

And there is the real problem. You don't need to "help" him see any of this is a problem. You need to tell him it IS a problem and he solves it or as shown above, you do. You're being what is normally called around here as a doormat. Someone who is being walked all over on. And sadly you're allowing it. Find you inner ENOUGH mode and put a stop to it.

To stop my things from being taken and used and disappeared, by a then SD9 and SD11 - and a DH that didn't do a thing about it because what was the big deal, I locked my stuff up. Done. Everything moved from the bath that I cared about (makeup, hair products, skin care, brush, etc.) to the bedroom. They were younger and for some reason our bedroom was not encroached upon for the most part so out of sight out of mind.

I have locks on several cupboards in the laundry room now. Things I don't want messed with are there. DH does NOT know where the keys are.

Lock up your stuff, including your extra vehicle car key right now.
Your DH doesn't do anything then you do. No discussion. Just do it.
I have zero regrets.

do you share a bathroom with skid, or did SD come into your bathroom and took your stuff then went to her bathroom..

Regardless - start locking up your stuff and do not be shy, if you catch her taking your stuff, stare her down and tell her.. little girl I am not your mother keep your hands off my stuff understood... (she's an adult )

SS using your spare vehicle - why is the keys available for them, simply take the keys and lock them up, sorry no child will ever simply take a vehicle in my yard that belongs to me without asking, and if he asks - the answer is NO, my vehicle not yours...

SD17 taking Dad's camera.. not my problem, but I have an issue with her coming into my room, I would simply start locking my room, tuff DH.. if I'm not there the room is locked, and it will remain this way till you can teach your children to stay out of the room, it's my privacy not for them to contaminate

You can tell the skids in a real bitch way to leave your stuff alone, and BD's for that matter, no need to be nice...
unfortunately they can ignore you and there's nothing you can do about it, the only consequence would be, you start locking things up and taking all car keys away...