Actually I wonder if your mother has given any thought to how akward it might be for her DiL. The two kids being similar age, but uncle - nephew. Akward. This is going to be a big part of their lives going forward and your wife knows this.

Maybe your mom could try to be understanding and let her have her baby shower without her presence there to upstage her. Which it probably would atleast to some degree. Im sure your mother would not want that for your wife. Explain to her the chance of that happening and ask her, if she is sure she has to be there with that in mind?

I donnu. I would probably back the wife if I was in your shoes. She and your baby is your family unit now. Moms needs and emotions are not your first priority. It can be hard to get your head around that. But its not got anything to do with logic. In this case its got to do with emotional support. And being there emotionally for your pregnant, and very emotional wife. Your moms going to have to get her support elsewhere. Not from you on this one. And dont show a united mother-son front. Its actually borderline inapropriate that she goes to you, after Dil tells her she doesnt want her there. She should be respecting that boundary. Because her emotions are not priority at DiLs babyshower. And she shouldnt be pushing to get her way.