Tell them that the current living situation isn't working for you, so you'd like them to start looking for a new place to live within the next 1-3 months (I'd give them a hard deadline, but it just depends on how fast you think they can convince some other poor person to let them crash with them). But they are grown ass adults and they have no reason to change since you are basically providing everything for them - I still don't get why the nephew's girlfriend is living with you?
Why can't all 4 of these adults get a job, even at fast food, and rent a two bedroom apartment together? They already live together, so with 4 of them working, they'll be able to afford it and will actually have to be responsible for themselves then (or risk being on the street).
I'd also sit them all down and tell them that they are adults, and they need to get their act together as they are running out of options. It is time to grow up and move on. They are VERY ungrateful if can't even contribute to the household that is supporting them by performing basic chores let alone hold down a job and save money for their future. And leaning so heavily on friends and family in this manner is a sure ticket to ruining their relationships and makes you all less likely to be there for them in the future when they might really be in dire straights.
But do give them a short deadline to get out of your house. They have no responsibility, no reason to work or do even the most basic of things even as a favor or to show how they appreciate what you're doing for them, since they are so lazy and disrespectful and don't even understand how they should be kissing your feet and cleaning the hell out of your house and doing whatever they can to show they appreciate you and your husband providing them with shelter, food, running water. You are part of the problem. In short, they need you to stop being their enabler.
I know that you care about all these people, but you are doing more harm than good right now. They are stupid, lazy jerks basically (and many young adults are, and they may grow out of it given time and the need to support themselves). You are seen as a mealticket and crashpad for them right now, and they don't respect you or what you're doing for them. It is time for some tough love - and tell them to get out and make it in the real world.
You can just tell them nicely, that in X days/month they need to be out of your house because this is not good for them or for you. They can't stay children forever and it is time to grow up and be responsible for themselves, and the longer they delay, the worse it will be for them. And tell them you and your husband can't afford to support them much longer and there are serious money issues happening, and the house situation is becoming a huge stressor for you and you are very sad and worried all the time. Even it none of that is true - if you feel the need to lie to get them to see it's time to move on and it will save you from having screaming fights, then say whatever you need to say to get them the hell out of your house and don't ever let them back in for anything.
