Where is the money coming from? If it's the MOB that is paying you, it might be tough to avoid her. I've heard from photographers that will say (or have in their contract) that they will only obey the bride & groom (or only the bride etc). This way, when the MOB does start ordering you around, you can say that you will only take orders from the bride. Whether this actually works to dissuade an annoying MOB or not...probably depends on the individuals involved.
I think the biggest 'defense' for you, is to have a good solid contract, that has clauses to address many of the foreseeable problems. In this case, it might also be a good idea to have a good list or itinerary as part of the contract. That way, you can try to set out what the bride wants, and if the MOB starts changing things, you can say 'That's not what is in the contract'.
If you have booked the wedding already, it may be too late to include stuff like this...so you may just have to chalk it up as a learning experience and make changes for next time. Who knows, maybe everything will be fine.
A lot of photographers, especially the busier ones, would be inclined to turn down a wedding where they could sense trouble. The initial meeting is not only for them to interview you, it's for you to interview them as well. If there is a good chance you will book the day if you turn it down, then maybe that's the best solution (in that situation). Actually turning them away, can be a hard thing to tell them...so you would need to word it carefully and say something along the lines of 'I don't think that my style is right for you, and you would be better off with a different photographer' and maybe give them a few other photographers to call.