Sounds like the real issues are: she is bored, she really wants to do something nice for you, and she likes hosting. I'd keep in mind that "the shower" is not the real issue and tackle the actual concerns separately. She sounds like a sweet woman, and I'd be willing to bet she doesn't have a daughter close by or one that she is close with.
Let her play surrogate mom without the shower; continue to insist you don't have time or don't have a guest list, and make a big effort to include her in your other children's lives in the meantime. Let her help plan one of their birthdays if there's one coming up or suggest she help you host a Valentine's day party for your neighbors and their families if you do things like that. Or ask for her help shopping for baby or making freezer meals. After all, what she really wants is to be helpful and for you to pay attention to her, so just guide her in a direction that's useful instead of stressful!