Sorry for the long message....
I don't think some people on here have read all replies by me or others. Thank (some of) you for your helpful replies. After posting my first message and frustrations, I have worked out that the issue I have is the dog, and that it isn't fully house trained, is a young dog (below 12 months is a puppy???) and could bite a 6 month old child (thinking its another puppy playmate?), destroys furniture in its own house, and would be more of a problem than anything else mentioned from the original post. I think I was trying to think of other excuses for them not to come, because of the dog (I.e. they weren't exactly invited) as a way to not upset them that their dog being in our house is the issue.
2 roasts doesn't alway mean 2 meats (cook one and 2 lots/days of freshly cooked veg), I have worked out that's not my issue. I think my anxiety levels went through the roof when I first got told this, and I came on here blurting out the situation, frantically looking for support and advice to soothe not only myself but to quickly defuse the notifying them of dog rule so they have time to change their plans.
Bil is a pain, and so are the in laws sometimes! I accept them and have always welcomed them. DH has always tried to see them on either 25th or 26th but it wasnt always fulfilled. I have not welcomed the dog before as its not even 12 months old yet. They have only brought the dog here once and we went straight out for a walk with bubba and dog then they went home. (I think that was our first way of trying to avoid the you cant bring the dog in issue, when bubba was a lot smaller). Dog will not be welcome at xmas. We like dogs, but do not own one and do not want one that clearly is not house trained, to pss and sht everywhere with 6 month old DS crawling around and DS putting everything he finds in his mouth. Mil dislikes her sons dog and other dogs for that matter and will not allow the dog in her house, and fil owns a cat and is more of a cat person.
Forgot to add, fil lives hours away, and we will already be hosting him overnight here on boxing day night. This is fine, and has happened several times since DS was born. He's never an problem when hes here.
Spoke to DH about it again yesterday and DH agrees with me (is on my side about the dog), but is worried bil will not come, and hes desperate to tell bil nicely, to keep some sort of relationship with his brother. They have a large age gap and weren't close as kids, but have been close since I've been around (encouraging them to hang out or come over or go to theirs). DH is worried that this dog situation might stop the status quo of their brotherly relationship, or take it back to barely anything.
We have already established that they have stated they are bringing the dog. We are going to ask them if they are staying at her parents over Xmas day night. If the answer is yes, that means they've taken the dog with them overnight.
That will mean we are going down the "ok great! Lunch is at (12?) until (2pm?) So see you at 12!"
If the answer is no, or if they ask why they arent staying longer...
"Unfortunately, due to DS being so young and rolling/crawling around, im afraid it might be best the dog is left at home, or if you bring it, it is welcome to explore our garden while we eat!"