I've name changed as some people in real life know my posting name.
MIL has a dog, about six years old now, that she treats like a baby. I find it a bit irritating at times, but generally it doesn't affect anyone so none of my business.
However, recently the dog has developed some toilet issues. The vet and dog behaviourist, I think that's what she's called, have both agreed it is a behavioural issue rather than illness. She was telling me yesterday about how they took the dog to a restaurant and dog pooed twice indoors. She said she was worried that the owner would see and ask them to leave but luckily she picked them up in time so the were able to stay for the rest of the meal! The dog had walked away from the table and it was near other people eating. I was quite shocked that they hadn't left after the first incident.
And last week she was complaining to me about how her sister won't let them visit with the dog until she can be sure he won't wee and poo in their house again. The sister has three dogs herself so should understand that he needs to be trained, apparently. On their last visit there he went to the toilet inside the house at least ten times, on carpet as well as hard floors, so sister was understandably a bit upset. MIL didn't inform her sister about the problems before the visit and refuses to use absorbent puppy training pads as it will embarrass the dog 
MIL says that she didn't think it was a problem as her sister has dogs (who are fully house trained!).
Now PIL are coming to lunch next weekend and MIL is expecting to bring the dog. This is their first visit to us in our new home together and we did not invite the dog. We live in a carpeted flat with no outdoor space. I assumed, rather foolishly, that if we hadn't specifically invited the dog they would not bring him. They live less than half an hour away and often leave him for longer than their visit will take when they go to other events. There is no reason they can't leave him. I tried to explain to MIL that we don't want the dog going to the toilet in our new flat and that we have nowhere outside for him to go but she thinks we are being very unreasonable and as family we have a responsibility to help him with his behavioural issues.
DP agrees with me but I'll admit we are both a bit touchy after she compared the dog's toilet issues to my recent diagnosis with a life limiting genetic condition so I don't know if we are being unreasonable. None of my family have dogs so I'm not used to these situations. AIBU to not allow the dog in our flat?