Geez, it's a wonder your husband would even WANT to be alone with this girl. Something is not right with her. Why on Earth would she want him to sleep in her bed with her? A twin, no doubt.

What does he say to her when she specifically asks him NOT to bring you?

I wouldn't want to go, either, but I might just have to, like you said, to make a statement (that your husband should have already verbally made).

If he goes alone, he should take an air mattress and a pump. Or, stay in a hotel.

Honestly, my husband would probably not go without me, even if I insisted. He just wouldn't go, if his daughter made him that uncomfortable.

Go with.  And stay in a hotel.  This young woman obviously has problems. 

Your DH is ready to disengage FROM HIS OWN DAUGHTER which has red flags all over it.  There is something very twisted going on, but only your SD can get help for herself.  Which she probably won't do.

IMO, your DH should never visit her alone from now on.  Suggest any visits be in a public place (restaurant) and make sure that you sit next to him at all times.  

If she begins pulling any of her inappropriae behavior, you are perfectly within your rights to call her on it.  Tell her exactly what she is doing, how inappropriate it is (for an adult female), and that she should seek professional help.  And make SURE your DH backs you up!  Figure out your gameplan of dealing with her, and the two of you should not waiver. 

WT! I must live in a bubble or have been lucky to never deal with something like this. None of this is normal. Never leave this girl alone with your husband.

My SD was very similar. Finding her thongs on the bathroom floor, bright red bras over the shower rod, she'd comment on how "cute" her father is making everyone cringe, cuddle with him on the sofa...the whole bit. It always seemed desperate and needy more than anything. 

I never thought it was sexual on my husband's part, and really not on SD's part either. Flirting is how she communicates with all men -- her cousins, her dad, the cop writing her a ticket, her uncles. She's an only child and the only girl in two generations making it all the more "normal" for her to be the coquette among her harem. I've often wondered if it got worse after I came on the scene and she felt threatened. She's be hanging all over him and I'd just be sitting across the room feeling like I should look away.

I once made a comment that my parrot, who was very attached to my husband, screamed so much when SD was around because they were sitting on the sofa like lovers -- it did not go over very well with SD. It was as if I called her out on something everyone else had chosen to ignore.

Husband and I addressed this and other things therapy. In short, the therapist told my husband he has to put a stop to the physical stuff. For example, the therapist told him never to sit on the sofa when SD is at the house so she can't snuggle up to him and end up on his lap. He was told to always specifically choose a chair so she has no access.

It's ridiculous to think he had to be so strategic about where he put his ass, but it worked. Your husband can do the same: position himself physically to be out of her reach. This "rejection" of the physical with SD can start the ball rolling on breaking the emotional enmeshment.

A similar situation happened with my SD. She was fine, but territorial when I was dating her dad for two years. When she went off to college, we got engaged and then married. After our marriage, she moved in with us and started doing weird things that started out as always wanted to sit next to her dad, and have private jokes and conversations, to flirting and then overtly sexual behavior. It did not stop until I kicked her out and DH became disgusted with her non-stop flirting. He has to let her know that their relationship is father / daughter and not dating and mini-wife. 

He needs to skip the trip or take you to show her that you are not divided and that you are his WIFE and top priority. It took us two years to straighten out this mess she started and I wish I had put a stop to it sooner. I wanted to give them lots of alone time, as I thought that was what she needed. But in reality, she somehow wanted to show her dad that she could be the wife and not me. After lots of work and boundaries, things are getting better but it’s her dad that must set the boundaries, not you. And for a long period of time, you should always be with your husband to show her that you are a team and to support him. My husband was so appalled by his daughter’s sexual coming on to him that he froze. He needed support when he was with her to stop the weird behavior. Once she sees that is doesn’t work, she will stop. But it takes time, clear boundaries and consistency from both of you. 

I would treat the entire issue, as you would any other woman that is coming on to your DH.  If your DH is uncomfortable with his DD and her actions, tell him you can stay in a hotel and you will go with him anytime he wants you to go.   

Disengage and ignore this sick lady.  Support your DH as long as he is in your corner.

What is the purpose of the upcoming visit in 3 months?  If he's going there to help her with something I would suggest DH let her know he can't make it and hire it out to a local handyman, mover, or whatever she needs.  Just visiting, catching up, seeing her? Make it a short passing through day trip where you all grab lunch, see a sight and are on your way to a romantic couples destination.  Avoid her apartment at all costs!  

SD obviously has enmeshment problems that are likely fueled by BM.  She claims she was molested by a family member so maybe in her mind it's okay for SD to be overtly sexual with her father, especially since it's serving the purpose of driving a wedge between the two of you.  

ss33 has come over and sat with thighs touching DH. It's like he has to take up the space so I can't sit next to my husband.  He wins, I leave the room and let DH deal with his emotionally crippled spawn.