You need to talk to your hubby and he shouldn't have made decisions about the son moving in without discussing it with you. There need to be some rules for the 20 year old while he's living in your house. You and your hubby need to set up the rules together and enforce them together. If he's defensive about his son this is going to be impossible and I don't see things changing.

I don't see why what seems a perfectly valid concern to me is attracting such criticism. The OP doesn't appear to be saying the ss20 can't stay there, all she's saying is that he needs to stop being a lazy, selfish pain in the ass, clean up after himself and as the third ADULT in the house, do his share of chores just like the other 2 adults in the house already do. He also needs to get off his lazy butt and get a job and pay his way. Doesn't matter whether it's dirty bowls, glasses or mess from making a 3-course meal left in the kitchen, it's still totally unacceptable that OP leaves the place clean and tidy and comes home after working all day to a mess made by another adult who's done nothing else all day but eat, sleep and play stupid computer games! He's getting housed, fed and presumably getting rides and his laundry etc done for him for free, at 20 years old when he's more than capable of contributing financially and otherwise!
I totally get what OP is saying about the example this is setting to the younger kids - how many little boys look up to their big brothers and want to do everything they do? And as for not giving the 12yo girl the privacy she deserves, that is totally unacceptable and the dad of this 20 year old loser needs to tell him in no uncertain terms he never, ever goes in the girl's room again. The ss's reasons for doing this may be totally innocent and he may not even have thought about how it could be interpreted, but whatever his motivation, it sounds like the OP is aware and is, very wisely, keeping an open mind at the moment but at the same time is keeping an eye out for any signs of the ss doing anything inappropriate towards her daughter.
The father also needs to be laying it on the line that he expects his son to get a job and pay his way and do his share of jobs around the house, and if he doesn't mend his ways before a set deadline, he has to go and then he has to stick to his guns if/when his son fails to do as asked.
I think part of the problem here is the father of this so-called "adult". Unless he stops being such a soft touch and lays some rules down for his son, I'm afraid the OP is probably fighting a losing battle. All she can do is make it clear to her husband that the current situation is unacceptable, tell him what she expects of her SS for him to be able to continue living there (ie perfectly reasonable requirements of get a job, do his share of household tasks and respect her daughter's privacy), tell him it needs to be done by a deadline and that if it doesn't happen, ss will have to leave otherwise she will and mean it.

Couldn't read the whole thread.

Sorry you got a lot of lint in there. Ignore it.

You did not raise this sloth, you should not have to suffer the consequences of how he was raised.

Try to have a loving heart to heart with your dh about setting up a plan to launch for this critter. It starts with expectations around the house and positive steps toward doing something with his life. No, 2 college classes a semester doesn't cut it.

If he only wants to take 2 classes, he should be able to be fully self supporting. Applebee's, Walmart, Jack-in-the-Box, grab those applications and start working, boy. Untold legions of people put themselves through school that way.

If he wants to stay in dad's house he has to contribute to the household via all the rules you and dh are going to set up AND he has to go to college full time.

I don't see how you can survive without one of these two plans.

I would draw a line in the sand over this. This (young man) was never your problem. You are willing to work with your husband in solving it but you are not willing to throw your own kids nor yourself under the bus for it. If you have to, I would move myself and my children out at some desperate point. Let dad "raise" his legal adult. You can remain married apart. You have to make dh understand how serious this is. And then you have to build your life.