You're going to have to go crazy. Lay it all out to DH and SD24, leaving nothing out. Tell her she has 6 months to find her own place. Is she receiving CS from the 3yo's father? Does she have enough income to pay rent? If she is working FT, she needs to get her butt out the door already. She will never start doing her fair share around the house because DH is there. Give her an ultimatum. 24 is way too old to be sticking around at the homestead.

If DH gives you any lip, tell him you've been patient long enough. For TWO people (incl the 3yo). Pack a bag and tell him you are leaving, even if you only go stay at a hotel for a week. It's time to drop the bomb if you want to see anything change. Unfortunately, it usually comes down to the SP speaking up, LODLY, in order for anything to change. Good luck and keep us posted!

~ Moon

IF she contributed by cleaning the house properly for you .. she could stay
IF she was paying off your car in lieu of rent ... she could stay
IF she had some frightful student loan and there was an end in sight because she had it all worked out to move our by April next year and 'thanks Dad and SMom for letting me pay this off sooner than i ever could alone" .. she could stay.

So instead, tell DH that as his priority is enabling his daughter to live rent-free without growing up you are moving out to your own place. He is welcome to visit you there, you aren't looking at a divorce, just a better living arrangement. And as the filth overruns his life and he sees what a nice place you have he may change his mind.
Don't hold your breath thought. Obviously being a father is more important to him than being a husband.

The son will move in with you shortly. The BM's revenge will be complete.

The $700 a month CS to the son who lives with BM has nothing to do with your situation going on in your home.

SD is now an RN with a fulltime job. It's time she rents a small apartment for her and her child. No ifs or buts. You and DH went above and beyond by supporting her while she made her way to becoming self supporting. Well, she's made it, so out she goes. She could be a neat freak and the most helpful housemate in the world...it doesn't matter...it's simply time to go. There's no reason for this self sufficient woman and her son to now still be living with her father in your home. None.

Don't fight with Dh about what a slob and lazy she or how she spends her cash . None of this is necessary and will make 'Daddy come out defending his 'little girl'. Now it should be about holding his daughter back from launching and being the woman she's capable of being. Out into the real world she needs to go. You have no intentions of continuing to support and enable a full grown woman who is quit capable of being independent. A home isn't meant to house a person who just would rather keep living with 'Daddy' and Sm and let them keep supporting her.

It doesn't have to be confrontational with DH or with SD. It simply is. DH needs to hand his daughter the rental ads, a budget planning book and a hug goodbye.

Time for her to go. In the time she has been with you she has had a child and gotten a degree and employment. Time for her to be a grown up and move out. You are not only supporting her, but also her child which she needs to be doing and she can be going after her child's father for support. Not one reason for you to be doing it.
Good luck to you.

3 months notice. Period.

One of my favorite quotes that I relayed to DH MANY times when I was in the same situation you're in:

"One must not do for another what they could and should be doing for themself"
Abraham Lincoln

Go read Stepdown's blog. She recently left her SO over his adult daughter living with them.

If my SD brings a felon around our house, especially after age 18, and my DH is not united with me on plan to keep him out, including tossing girlie, I will move out. Period.

If SD gave welfare benefits to her mom while living off us, I wouldn't move out. I would change the locks on the house and invest in a security system. DH can come inside. Her stuff is either sent to BM's or confiscated to me for expenses. She ain't comin' back. Then, if feasible, I would report the welfare fraud.

If your DH is such a great parent, how come he has a kid consorting with felons and not only ON welfare but committing welfare fraud? He needs a reality check.

RN'S make decent money. Enough to support her own place and be 100% financially independent. I would never have lasted 7 years.

Even the lowest paid RN I know makes well over 40k

More than enough to move out,

Start charging her rent, take that rent and put it in a secret account. In 6 months there will be enough in the secret account for your SD to pay first months rent and a security deposit. It's time for her to get out and start her new life. If you can't get DH to stand with you on this, you have no choice but to leave.