Peppina, I have been in your situation, MIL has a very big untrained dog that she treats like her baby. He jumps up, isn't (IMO) adequately exercised (because she cannot control him while out) and is frankly an accident waiting to happen.
I spoke to her during my pregnancy about the safety of my baby around the dog and also my safety while pg as he jumps up so much and is so big (he weighs 9stone and can put his paws on my 6ft 4 DH) and I was assured he would be kept completely out of the room while we were there, he would never hurt anyone, baby would be safe etc etc...
A couple of months later, while I was 7 months pg, she let him in to the living room while we were there, because she "felt sorry for him", this was without my knowledge (or I would've immediately left), he went to jump up on me but thankfully I was able to move out of the way quick enough and I wasn't hurt.
From that point, I have never returned to her house. DS is now 3.5.
MIL makes a lot of noises about how terrible her life is, her friends, family and her grandson do not visit her house because of the dog but her complete disregard to others safety makes me lack any sympathy for her.
My choice to not visit there and a ban on DS ever going there hasn't been popular and it caused a lot of arguments but I have stuck to my guns and after a lot of weeping and wailing, she doesn't mention it to me any more, just comes to us when she wants to see DS.
It's a sad situation for all really, DS doesn't see DH family as much as he would if it weren't for the dog (they are a 20min drive from us so not far), and MIL is missing out on having the relationship with DS that my parents have. I also contrary to what MIL thinks, really feel for the dog, if it had been properly trained and socialised it would've had a much better life.
I blame MIL for the whole sorry mess really.