Even before I went away to university, I had the box room (I'm eldest, with two younger brothers that shared a room not that much bigger than mine tbh).
Now, this room truly was a box room. Smaller than a double bed - it fit my single, a bookcase and a small chest of drawers, and not especially comfortably.
When I first moved out for university, it was cleared out and a futon moved in for me for holidays. Later, a bed was moved back in, which was mine again for the year or so I lived at home before moving out with my partner. By this point, the only furniture in there was the bed, the bookcase, under bed drawers and some wall shelves, so I had a bit more floorspace than when I was a teenager.
In that time, my partner stayed with me numerous times, just about managing on a futon mattress on the floor or squeezing into the single together. It's more than doable.
Nowadays, that room is my youngest DB's, so staying at my parents' is never ideal (they have a futon in the conservatory, but it's always either too hot or too cold depending on the time of year). We make it work.
If your youngest DD has had the box room for 12 years, it's definitely her turn for space.