I think a phone call is critical - anything you attempt to convey via email or text puts you at high risk for having your tone or meaning misinterpreted. It will cause a lot more drama if done in writing than if you actually talk to her and can work it out that way.
There must have been some miscommunication along the way - either she thinks she asked you and didn't, or she asked in a passive/aggressive way so you didn't even realize you were being asked. Or, maybe you said something that she interpreted to mean you would help, even if that's not actually what you said or meant. In any case, by talking on the phone, you can work it out.
If you're attending the shower, personally, I don't think it would be a big deal to bring something small or simple. Maybe just offer to pick up a cake or something. Trust me, I totally get where you're coming from and don't blame you one bit for being annoyed or frustrated. I don't like being asked to help host a shower when I didn't offer or plan to. But, having been the host before, I do appreciate help, especially from family members or friends who are closest to the parents-to-be. So, for that reason, I'd probably offer something like dessert since it doesn't require too much effort on my part. I enjoy baking or could easily order and pick up a cake. A nice cake is usually available for under $25 from Costco or a grocery store - it doesn't have to come from a fancy bakery.
I don't think you and your sisters necessarily need to all do the same thing either. If one of you agrees to help and another doesn't, that's ok too. Each of you should do what's best for you.