My SD stopped coming over for good when she was 16 (big drama that I will spare you) but at 18, your SD should be learning some responsibility and it's up to your DH to teach her.

I'm pretty sure my DH would have been okay with his daughter still coming for visitation at 18 but I know that he would have used that time to try to teach her about being an adult.

Eliminate a place for her to sleep.  When she asks why you tell since she aged out of the CO and she hasn't been over you revamped your situation a bit.  Sometimes you have to be blunt with these skids and tell them they are adults now and the rules change.  Sometimes you need to do that with spouses too.   I'd take this time to discuss this with SD and DH and what to expect going forward. SS is still under the CO so he sticks with that.  
 

If you want nothing to change, keep quiet.  

He can visit if and when he wishes.  It goes the same in the other direction. We visit him whenever the opportunity arrises.  

This is a family thing. My parents have the same deal with my brother and our families.  The GKs can show up any time.  We all make an effort to visit and spend time together when circumstances get us close enough to fit in a drop in.

Though we all make sure to pre-notify with some reasonable heads up if possible.

I absolutely don't think you should be paying for bills just because SD decided she didn't want to adult for the summer and has no $. She is 18, well past time to take some responsibility and learn you can't play all the time and pay for nothing. 

Many parents will support their children and house them.. pay their bills (like medical). even after 18.. especially if they are planning on going to school... it does sound like she does at least also work some and doesn't expect her parents to pay for all her extra curricular fun stuff.. so be thankful for small blessings there...lol.

If your DH is expecting you to personally subsidize her because he doesn't earn enough? that's a different issue... you don't get to be generous with other people's resources.  

You also need to have a discussion about what exactly is the level of support he will provide.. and what the conditions are for that support.. and how long he plans on doing it.

For DH and I, our children didn't stop being our children magically at age 18.  Our home is their home.  If they give us a heads up that they're coming they're welcome to spend a minimum of 3 days no matter how old they are. And if there is some emergency, they're welcome to stay longer, even if they're full on adults.  

If they're still in school, then they're welcome to move back in with us if they need to (and each of them did at one point!, but my kids and SKs are awesome, contributing members of a household and I adore them all).  As long as they're making continuous steps toward adulthood I'm happy to help (not enable), and enjoy their company.

As for what you pay, well that's up to each family, but if she's paying for all her recreation, planning on going back to school and in general going in the right direction of course I would pay medical, likely some contribution to food and clothing as well.  

My DH is leaning the same way - I am sure he will welcome any visits from SD"s after they age out. I am not so happy about that. Maybe its more important to set some house rules that she must abide by when she visits, so that the visits aren't so hard on you. It is your house, after all. What SD behaviors do you dread the most? Then devise a way to get those behaviors to not happen when she visits, or she will be told to leave. I know its hard when DH doesn't back you up, but you, too have the power to make everyone uncomfortable in your home. Why not try it?    What? No Wifi? Oh no! It must be on the fritz! What? The crap you left on the floor in the living room disappeared?  Are you SURE you left it there?      What? There is no food in the house? I guess you will have go grocery shopping then. LOL>