I really don't know how to deal with my mom, except I know it never feels good to hurt moms feelings, no matter how out of her mind she gets. However, it might be best to simply explain your position in a civil manner Keeping in mind that your own family is your priority now and she doesn't get any say in how you raise it. Be honest, but not mean or nasty and you will be a better person for it. And remember when she gets nuts simply tell her she misunderstood.
Keeping in mind I cant even get the upper hand with my own mom, and she's not even rich.
Just my.02