MIL + big dog

My in laws have a huge dog that barks aggressively at anyone she doesnt know well. Though shes never bit anyone yet, I dont trust her around my tiny defenseless baby. My mother in law ignores my wishes and at our last family gathering she shows me pictures of my little 4 month old propped up on the couch ALONE with the huge German Shepard just inches from his face. I told her I didn't like it and I didn't want the dog near him and she tells me they need to get to know each other... The dog is only in town for a little while and my son only goes to my in laws once a week for a few hours at most with his dad, there is no reason for them to "get to know eachother". My husband finally told her he didn't want the dog near our son either, so hopefully that makes a difference... but I don't feel like we are asking for too much!! I love dogs, but they're unpredictable and this one isn't ever around kids, and it doesn't need to be around mine!!