I am so very sorry that you are going through this, clearly you are heartbroken.
The decision before you is one of a household rule that you and your husband will be making for your family. Will you allow your daughter, or any subsequent children to inform you that they are “staying all night” with a boyfriend/girlfriend while they are living under your roof and continue to live with you? Whether one of your children drives across town to sleep over at someone’s house and comes home the next morning, or whether they fly to Europe to do it doesn’t matter really since it’s essentially the same thing. If your daughter goes with your approval then you need to be prepared for younger children to inform you they are going to spend the weekend at their boyfriend/girlfriends house and you can’t object or start changing the rules then it won’t be fair. Don’t be fooled that your children won’t know she shared a room! They are young now, they will figure it out when they are teenagers! After all it is the natural sibling thing to do to make sure things are fair, when your younger ones want to spend the weekend with a boyfriend you will hear all about your daughters trip to Europe I promise you that.
At this time your daughter sees nothing wrong with sex outside of marriage, living together, or taking a vacation and sharing a room. How are you going to feel looking at her vacation pictures when she gets back? Are you going to allow your younger children to look at her vacation pictures? What if she talks about how nice her hotel was? You have even more decisions to make than where she is going to live you need to decide how this trip is going to be presented to your younger ones.
As I said previously, your focus at this time has to be 100% on what is best for your younger children. It is unfair to them to get a watered-down example of the faith because you are fearful of upsetting your oldest child. She cannot hold your family’s faith hostage like that, even though she is not doing it on purpose, you need to hold on to your faith strong! Your love your oldest daughter, however because of her behavior and her telling you that she sees no problem of living together you are operating out of fear that you will drive her into living with her boyfriend. You cannot parent from fear, fear is not from God fear is from satan. You need to parent from a position of strength and Jesus is your strength!
If you decide to let your daughter go I understand why, however you must be prepared to live with the consequences that staying all night is okay when your children are adults living at your home. I am so sorry that you are going through this please be assured of my prayers.