Sis is away at college. She lives about 5 hours away and he lives about 10 hours from school in another direction. He paid a visit over the winter break. And that was the one time we've met.
We've had boyfriends over before, and done things that are usually "family things" that included boyfriends. This is just feels a little different, it has always been just the immediate family...to the extent that Dad gets upset if mom invited her mother or sibling to come along. So to invite someone who isn't yet related seems bizarre to me.
He wasn't invited by mom because of how long he and sis were together. He was actually invited last year too, but the invite was so last minute that sis told us immediately that he couldn't make it. At that point they'd only been together for about a month. The cabin is actually closer to my sister's school than to us, so I think that was how he ended up invited: "oh, he lives an hour away, why doesn't he come along? He's already right there".
If he'd just shown up to hang out each day I'm not sure I'd really mind.
I'm planning on speaking with mom later tonight. And maybe I won't bring up the fact that "he isn't family yet". When I'm going over it in my head, that is the part that seems to put my argument on the childish side, rather than the reasonable one.
While I completely understand you not being happy with this,, I'm going to respond from your mom's point of view.
Her college age DD is living 5 hours away. She has a new boyfriend that mom would like to get to know but because of distance, that's not possible. She has the opportunity to have the boyfriend spend time with the family at a cabin. She also gets the opportunity to convey to her DD that she is looking upon her as an adult by offering DD the opportunity to include her BF into the family get together.
I think it is fair for you to say "Mom, what is your plans for sleeping arrangements. I don't want to share a room with sis's BF and I think I'd be uncomfortable if he were sleeping on an air mattress in the common area." Then get a conversation with sis going. "Sis, I think it would be great for your BF to join us but I'm concerned about sleeping arrangements. I'm definately not sharing a room with him. And if he sleeps in the common area, is he going to be upset if he's trying to sleep in and Dad and I are up fixing coffee before going out hiking early in the morning?"
I also wouldn't bring up your reasoning of "he's not family yet". Would you feel any different about sharing space with him if Sis calls tomorrow with "Guess what, we just eloped"? Or if your mom had invited a male cousin you havent' seen in 6 years?
I would not broach the idea of you giving up the bedroom at all. You were invited first and accpeted based on having a private sleeping area.
I wouldn't worry about day time activities. Sis and BF will most likely plan some separate activities, giving you guys a break. And if the norm for your family is for everyone to retreat into a book at night, then do that. Don't feel you have to entertain him. If they are really thinking about long term, he needs to be aware of the habits of what could become his extended family.