Mother-in-law Hell

My husband and I have been married almost a year, and he has a 2-year old son from a previous relationship. Recently, the custody order allows us to have him for a long weekend per month. A week before he is supposed to stay with us, my husband tells me he wants he, his son, and myself to stay over at his mother’s house while he visits. I then learn his mother called him that morning, saying that she doesn’t feel it is safe for a small child in our home. Keep in mind, hers is infested with rats and smells marinated in cat pee. Mine is spotless and child-proofed. She also refuses to let me have the child’s mother’s cell phone number. I just want it to show I’m being responsible. How do I side-step this showdown waiting to happen? She and I really aren’t close, but it isn’t a lack of my trying to form a relationship. I just wish I could avoid interacting with her all together.