ESH. I’m going to throw a different perspective in here. I definitely think it’s wrong that your mother and brother pressured you into this. They should have found other accommodations and let it go if your offer wasn’t suitable and you objected to their request.

However, if I were in the girlfriend’s shoes, I would not be comfortable sleeping in a communal area in a home where there will presumably be strangers utilizing the space, and it has absolutely nothing to do with the desire to have sex. This sort of falls into the idea of women carrying keys in their fist when they walk alone at night... certainly predators are the vast minority, but I would be far more comfortable in a room where I could presumably lock a door or sleep comfortably in whatever clothes I typically wear.

I’m sure that you’re a stand up guy, as your brother can probably vouch for that, but the same can’t be blindly trusted of everyone you live with. Also, I’m operating off of the assumption that his girlfriend has no preexisting relationship with these roommates of yours.

It is 110% okay for you to offer them the couch and not your room and to stand by that, but then to change plans at the last minute and not see why there may be more to it than them getting laid isn’t.