(((Lizzaayy))) - I think there will be enough people that are VERY supportive of you at your baby shower, and it sounds as if you get along with the aunt, that it will be okay. You've got good boundaries, and if she doesn't know he's using, or believes his quacking, that's on her. You can just say something like "he's known, all along, what he needs to do to be involved in his son's life" and leave it at that. If she tries to get more, just tell her you'd really rather not get into it.
You have that right, you have boundaries, and I think it will be fine. I kept in touch with my XABF's mom, until her death, long after he and I split up. Of course, there was no child involved, but she was a HUGE source of support and love for me. Yes, she loved her son, but she loved me, too, and wanted what was best for me. His aunt may turn out to be someone like her...who knows?
Hugs and prayers,
Amy