I think I agree with Lu about this.
I personally just don't see the issue...I am not one of those parents though that believes every decision needs to be vetted by me. Grandparents do all sorts of things without asking parents, and so they should - they are grandparents! What harm does it do you if they open a bank account? Why should it be anything to do with you if they want to open an account for their granddaughter?
Sorry but to me it doesnt sound like they are trying to control things, it sounds like you are scared of losing control and giving an inch.

I too though have a massive issue with your DHs response of "its over then" THAT is what I think you should be concerned about...the lack of support and unity in front of your PIL - disagreeing with you is one thing, but you do not pick sides in front of others

jmo though