Personally, I would suck it up. To a point. I would be honest and tell her that you are really busy and time is an issue, but that you'd like to co-host w/ someone else. Has anyone else offered?
I would just expect that she'd probably be really hurt if you say "sorry- not doing it". yes, a shower is a gift and to a point, she's rude to assume. But based on your history, I'm not shocked at her assumptions.
If she's done a lot for you and your kids over the years, and now here it is- her turn and you say "oh, oops, sorry. Too busy", I could see her being really hurt.