I just wanted to quote this (yes I know quoting is un-MN but I think this is the best post on the thread and I totally agree with it)
"I'm an evangelical Christian too, and for me, that means no sex before marriage... But I still think YANBU!
Because while I don't believe in it, I don't expect those who don't share my beliefs to do the same...
If it were her house, then maybe she might have some kind if say, but even then, if you're on your third DC then she'd still be being silly. But she can't go around enforcing her views everywhere she chooses to sleep - does she check that every other guest in a hotel is married to the person they're sharing a bed with before checking in?
There are also your other DCs to consider - what will they think to have mummy and daddy in separate rooms?
She needs to accept that you are a family - maybe not set up in the way she would have chosen but a family nonetheless. To suggest otherwise is unkind, frankly.
Maybe if she showed a little more grace and a little less judgement then her faith might be seen as the positive thing that I believe it to be.
For what it's worth my DSIL is a lesbian, and currently pregnant with twins with her partner. Yes, it leads to some delicate discussions with our DCs, but they live together, are a family, and if they stayed at ours they'd be in the same room, let alone if we were anywhere else.
Sometimes, there are things others do, that while you may not have chosen the same path yourself, you accept it because you love them, and to do otherwise wouldn't be loving, or graceful, or kind, and that's the most important thing.
I also think you've been very patient, OP.
Ignore her and carry on. If she wants to sleep elsewhere then that's her call. Pitch her a tent in the garden..."
well done db
OP I hope you have a good Christmas however things work out with SIL.