Dh sounds like he might be fighting your ds's diagnosis still, living in the fantasy that he can still be "normal," kwim? I would consent to going to visit MIL only if she agreed to board the dog at a friend's house or a kennel for the duration of your visit. Yes, your ds does need to learn to cope with a barking dog. But not at a visit to grandma's house that is *supposed* to be pleasant. Grandma's house should be a "safe place" where he doesn't have to force himself to cope with threatening situations. JMHO.