I agree with Balloonslayer, I also think the OP has been shielded from the financial realities of life, as the DPs parents have been subsidising their relationship for years, they have lived together as child/adults, while the DPs parents pay at least his share of their rent and expenses.
I didnt get a grant either, I had 3 jobs (at the same time) to pay my way through uni. The parents didnt have to subsidise him, they could have suggested he lived at home, he could have taken that option.
The parents are going to be out most of the time, they are at concerts etc.
They just want somewhere to sleep, never mind the attitude to the SIL.
The parents clearly lead active and independent lives.
Not many couples, without FT jobs, start out with 2 double bedrooms and an ensuite, nice for the OP that they can, but its clearly with the help of the parents. Who is paying months rent up front and bond? Or is that being waved.
I too get the impression the OP things its fine for her family and not his.
You both sound spoilt, entitled and ungrateful to me, and in part because you dont seem to realise, a spare room, large enough to take a double bed and a desk, is a lot more than a lot of young couples starting out have, its far from tiny, but the OP doesnt see that.
I think the OP cares more about her cat than her DPs parents, who appear to me have been a money tree for the last few years, to ungrateful children.
I wonder, when it proves to be hard to find FT jobs, who will be subsidising the DP??
They dont have the right to demand to stay, but I dont think they have, I think they already had things booked and saw a chance to save on hotel costs, and it hasnt occurred to them they woudlnt feel welcome.
For all any one knows they may think the OP would be offended if they stayed elsewhere, I wouldnt want my mum to stay in a hotel around the corner from my house, and begrudging them the parking space in favour of a friend, words fail me.
I for 1 would never ever begrudge my mother this, even if it meant I had to sleep on a sofa.
Selfish in the extreme. Thats how I read the OPs posts.