I agree with the others about the training. It's the environment he's raised in that makes a dog dangerous, not the breed. Our neighbor has a pit bull mix. He's very intimidating-looking, but he's the friendliest dog I've ever met. And DH had a co-worker with a Rottweiler that he specifically trained to be kid-friendly. One of the things he did while training was periodically pull his food bowl away, talk to him, give him attention, and then give the bowl back. When the dog got used to him doing that, then he had his kids do it. As a result of this and the rest of his training, this dog grew up into a huge, scary-looking, gentle cuddly teddy bear that absolutely loves kids.
Best thing for your son and DIL to do is to find a trainer who can train the dog to not only be protective rather than jealous and resentful of the new family members, but also to be gentle with all kids, especially when the twins get older and start treating the dog like a jungle gym.